SuttaCentral
We all know (think) that Metta is important and indispensable on the path to enlightenment.
However, it is not ‘written’ as part of the eightfold path, so some argue that Metta is good, but not necessary to the path.
Could anyone please cite early Buddhist suttas where the Buddha clearly stated that without Metta, one cannot get enlightened?
If there are such suttas (I hope to see a big number), why isn’t Metta included in the Ariya Magga or the Noble (eightfold) Path?
Thank you. 
Actually, Metta is part of N8FP, and it is its second aspect, right intention/thought, samma sankappa.
MN141 The Analysis of the Truths
And what is right thought?
Thoughts of renunciation, good will, and harmlessness.
This is called right thought.Katamo cāvuso, sammāsaṅkappo?
Nekkhammasaṅkappo, abyāpādasaṅkappo, avihiṃsāsaṅkappo,
ayaṃ vuccatāvuso: ‘sammāsaṅkappo’.
Many great buddhist meditation teachers equates nekkhama-sankappa, abyapada-sankappa and avhimsa-sankappa with nekkhama-sankappa, metta-sankappa and karuna-sankappa.
Seems as abyapada-sankappa is pretty much synonymous to metta-sankappa. So renunciation, metta and compassion are main driving forces of entire buddhist path.
Perhaps someone more learned might confirm this 
With Metta 
Isnt non illwill / abyapada not necessarily synonymous with being kind . It just refer to not being “mad” inside or towards someone . It can be simply a “Neutral” feeling which is a state of non hostile . Being harmless does not mean being understanding , caring and concerns . It is just a state not being dangerous to others .
That’s not correct; right view opposes ignorance, and renunciation, non-ill will and harmlessness oppose desire and anger. That’s why right view and right resolve together comprise the wisdom component of the noble eightfold path. Desire and anger are emotional, while ignorance is a condition of mind.
Understanding the four noble truths in relation to one’s own life gives rise to renunciation:
"And how do you watch after others when watching after yourself? Through cultivating [the practice], through developing it, through pursuing it. This is how you watch after others when watching after yourself.
Understanding the four noble truths in relation to the lives of others gives rise to thoughts of non-ill will and harmlessness:
“And how do you watch after yourself when watching after others? Through endurance, through harmlessness, through a mind of goodwill, & through sympathy. This is how you watch after yourself when watching after others.”—SN 47.19
In Pali the prefix a- can mean “opposite of…” (paṭikkhepa) as well as “absence of…” / “remoteness from” (virahita).
In the case of abyāpāda and avihiṃsā the commentaries understand both senses of a- as possible but more often take it in the paṭikkhepa sense, making the two words synonymous with mettā and karuṇā.
bhante instead of abyāpāda
avihiṃsā why not directly state metta karuna as right intention .
IMO Metta is object of meditation to develop your Serenity and Insight, it helps to develop all factors of path, as most of us here know path leads to cessation.
Coming your question reason why metta is not part of 8 fold path, I don’t know why, but IMO they are closely related.
The argument is that with metta, one can attain Brahmavihara, and once this life expires, one will go to the Brahma heavenly realm for a long time but not yet enlightened.
The argument is that he wants to get enlightened and cannot find information in the suttas as to its being ‘crucial’ to enlightenment.
If the commentaries are right in treating them as synonyms of mettā and karuṇā, then one would be indicating the same referent regardless of which of the two dyads one preferred to use. I don’t think the commentaries ever explain why the Buddha preferred one set over the other on any given occasion. Perhaps there was already an established usage of speaking of abyāpāda-vitakka rather than mettā-vitakka and he was simply conforming to this.
- He abides pervading one quarter with a mind imbued with compassion…with a mind imbued with altruistic joy…with a mind imbued with equanimity, likewise the second, likewise the third, likewise the fourth; so above, below, around, and everywhere, and to all as to himself, he abides pervading the all-encompassing world with a mind imbued with equanimity, abundant, exalted, immeasurable, without hostility and without ill will.
He understands thus: ‘There is this, there is the inferior, there is the superior, and beyond there is an escape from this whole field of perception.’
[In other interpretation. He understands thus: ‘There is this(suffering), there is the inferior(origin of suffering), there is the superior(cessation of suffering), and beyond there is an escape from this whole field of perception (by following noble 8 fold path).’]
“When he knows and sees thus, his mind is liberated from the taint of sensual desire, from the taint of being, and from the taint of ignorance. When it is liberated there comes the knowledge: ‘It is liberated.’ He understands: ‘Birth is destroyed, the holy life has been lived, what had to be done has been done, there is no more coming to any state of being.’ [39] Bhikkhus, this bhikkhu is called one bathed with the inner bathing.”
MN 7 The Simile of the Cloth, Bhikku Bhodhi
There is a difference with practising right Intention i.e. non ill-will and doing metta meditation isnt it .
Metta is letting go of already existing ill-will, not allowing new ill-will to generate, generating loving-kindness, increasing already existing loving-kindness. In this way if you see it is Right Intentions & Right Effort yoked together.
“And what, bhikkhus, is wrong intention? The intention of sensual desire, the intention of ill will, and the intention of cruelty: this is wrong intention.”
MN 117
“One given to wrong intention has right intention by which to extinguish it.”
(MN 8)
“… this, friend, is the escape from ill will, namely, the liberation of the mind by loving-kindness.”
“… this, friend, is the escape from the thought of cruelty, namely, the liberation of the mind by compassion.”
(AN 6:13)
Well, perhaps one should consider the intention with which a particular kind of meditation is being done…? One could do any of the Rupa Jhanas or the Arupa Jhanas or one could meditate on the Aggregates or on the Sense bases or do a Death contemplation or practice Brahmaviharas… all of these are described in various places in the suttas as leading to a blissful abiding or to the end of rebirth viz Nibbana, depending on how they are developed further.
Consider the last paragraph of the Metta sutta
Snp 1.8
And while one stands and while one sits
or when one lies down still free from drowsiness,
one should be intent on this mindfulness—
this is divine abiding here they say.But when one lives quite free from any view,
is virtuous, with perfect insight won,
and greed for selfish desires let go,
one surely comes no more to be reborn.
IMHO, if one breaks it down technically, the whole point of all these different meditation techniques is to develop the spacious calm mindspace apart from the world of the senses either for its own sake; or so as to be able to see Reality for what it is - Anicca, Dukkha, Anatta…unhook the Mind from being constantly conditioned, develop dispassion, let go of greed and aversion, contemplate fading away …non attachment, the cessation of craving… which blossoms as the final letting go.
There is no single best technique… if there was, the Buddha would have had no need to speak of multiple techniques! Rather, selection of the correct technique depends on the overall as well as current moment bent of mind of the person. Practicing Metta (ie Brahmaviharas) is right up there, alongside Anapanasati and Satipathana, all of which used singly or in combination can take one to Nibbana.
If A do not hate B , this is not equivalent to A must love B .
You are assuming the very thing that needs proving, namely, that the a- prefix in abyāpāda and avihiṃsā denotes the mere absence of byāpāda and vihiṃsā. But you haven’t yet offered any reason why we should construe the prefix’s meaning this way, rather than as it was construed by the Pali commentators, along with the Indian pandits of several non-Theravādin schools.
There are two strands in the noble eightfold path, one being insight and the development of right view, the other tranquillity and the eradication of passion (anger and desire). These two thereby assist one another but always remain separate and opposite characteristics throughout the NEP, one active and connected to right effort, the other passive and connected to right concentration. Non-ill will and harmlessness are part of the tranquillity strand (awareness release), but like all factors are developed through the independent action of right effort guided by right view. It is seen that restlessness and ignorance are the last two fetters to be removed, tranquillity severing restlessness and insight, ignorance.
"These two qualities have a share in clear knowing. Which two? Tranquillity (samatha) & insight (vipassana).
"When tranquillity is developed, what purpose does it serve? The mind is developed. And when the mind is developed, what purpose does it serve? Passion is abandoned.
"When insight is developed, what purpose does it serve? Discernment is developed. And when discernment is developed, what purpose does it serve? Ignorance is abandoned.
"Defiled by passion, the mind is not released. Defiled by ignorance, discernment does not develop. Thus from the fading of passion is there awareness-release. From the fading of ignorance is there discernment-release.”—-AN 2.30
Greed, hate, and delusion are makers of signs.
Rāgo kho, bhante, nimittakaraṇo, doso nimittakaraṇo, moho nimittakaraṇo.
~SN41.7
Greed, hate and delusion are makers of signs for the heart. Strung along in to samsara.
When you attend properly on the ugly feature of things, greed doesn’t arise, or if it’s already arisen it’s given up.
Tassa asubhanimittaṃ yoniso manasi karoto anuppanno ceva rāgo nuppajjati uppanno ca rāgo pahīyati.This is the cause, this is the reason why greed doesn’t arise, or if it’s already arisen it’s given up.’
Ayaṃ kho, āvuso, hetu ayaṃ paccayo yena anuppanno ceva rāgo nuppajjati uppanno ca rāgo pahīyatī’ti.And if they ask, ‘What is the cause, what is the reason why hate doesn’t arise, or if it’s already arisen it’s given up?’
Ko panāvuso, hetu ko paccayo yena anuppanno ceva doso nuppajjati uppanno ca doso pahīyatī’ti?You should say: ‘The heart’s release by love.’
Mettā cetovimuttī”tissa vacanīyaṃ.When you attend properly on the heart’s release by love, hate doesn’t arise, or if it’s already arisen it’s given up.
Tassa mettaṃ cetovimuttiṃ yoniso manasi karoto anuppanno ceva doso nuppajjati uppanno ca doso pahīyati.This is the cause, this is the reason why hate doesn’t arise, or if it’s already arisen it’s given up.’
Ayaṃ kho, āvuso, hetu ayaṃ paccayo yena anuppanno ceva doso nuppajjati uppanno ca doso pahīyatī’ti.And if they ask, ‘What is the cause, what is the reason why delusion doesn’t arise, or if it’s already arisen it’s given up?’
Ko panāvuso, hetu ko paccayo yena anuppanno ceva moho nuppajjati uppanno ca moho pahīyatī’ti?You should say: ‘Proper attention.
Yonisomanasikāro”tissa vacanīyaṃ.When you attend properly, delusion doesn’t arise, or if it’s already arisen it’s given up.
Tassa yoniso manasi karoto anuppanno ceva moho nuppajjati uppanno ca moho pahīyati.This is the cause, this is the reason why delusion doesn’t arise, or if it’s already arisen it’s given up.’”
Ayaṃ kho, āvuso, hetu ayaṃ paccayo yena anuppanno vā moho nuppajjati uppanno ca moho pahīyatī’”ti.
AN3.68
The unshakable heart’s release is said to be the best kind of signless heart’s release.
Yāvatā kho, bhante, animittā cetovimuttiyo, akuppā tāsaṃ cetovimutti aggamakkhāyati.That unshakable heart’s release is empty of greed, hate, and delusion.
Sā kho pana akuppā cetovimutti suññā rāgena, suññā dosena, suññā mohena
SN41.7
N8fp is a path to end suffering , to develop goodwill is an escape from illwill ripens in attachment and does not lead to liberation . Right intention is a mind-set directing at abandoning craving illwill and cruelty regardless of developing goodwill or not .
Sentimentality regarding non-ill will is not profitable (SN 47.19).
Thanissaro:
“And what is lack of food for the arising of unarisen ill will, or for the growth & increase of ill will once it has arisen? There is awareness-release.[1] To foster appropriate attention to that: This is lack of food for the arising of unarisen ill will, or for the growth & increase of ill will once it has arisen.”—-SN 46.51
Note
Through good will, compassion, appreciation, or equanimity.
“Furthermore, there is the case where the mind of a monk, when attending to ill will, doesn’t leap up at ill will, doesn’t grow confident, steadfast, or released in ill will. But when attending to non-ill will, his mind leaps up at non-ill will, grows confident, steadfast, & released in non-ill will.”—AN 5.200
Apart from being a direct component of right resolve, non-ill will is represented in the path by equanimity, which is the culmination of the brahma viharas, and is a factor of awakening.