DN 2:
"Ajatasattu was the son of King Bimbisara of Magadha, one of the Buddha’s earliest followers. Urged on by Devadatta — the Buddha’s cousin, who wished to use Ajatasattu’s support in his bid to take over the Buddha’s position as head of the Sangha — Ajatasattu arranged for his father’s death so that he could secure his own position on the throne. As a result of this evil deed, he was destined not only to be killed by his own son — Udayibhadda (mentioned in the discourse) — but also to take immediate rebirth in one of the lowest regions of hell.
In this discourse, Ajatasattu visits the Buddha in hopes that the latter will bring some peace to his mind. The question he puts to the Buddha shows the limited level of his own understanding, so the Buddha patiently describes the steps of the training, beginning at a very basic level and gradually moving up, as a way of raising the king’s spiritual horizons. At the end of the talk, Ajatasattu takes refuge in the Triple Gem. Although his earlier deeds were so heavy that this expression of faith could have only limited consequences in the immediate present, the Commentary assures us that the king’s story would ultimately have a happy ending. After the Buddha’s death, he sponsored the First Council, at which a congress of arahant disciples produced the first standardized account of the Buddha’s teachings. As a result of the merit coming from this deed, Ajatasattu is destined — after his release from hell — to attain Awakening as a Private Buddha."—Thanissaro
Note on Ajātasattu’s Enlightenment
"If it is true that King Ajātasattu could have gained the Sotāpatti-Path Knowledge instantly but for his parricide, how can he become a Pacceka Buddha and attain Parinibbāna? If it is true that he will become a Paccekabuddha and attain Parinibbāna, how could he have gained the state of a sotāpanna? Enlightenment of a Paccekabuddha consists in the fulfilment of five things: (1) manussatta (a human life), (2) liṅga-sampatti (being a male), (3) vigatāsava-dassana (discernment leading to freedom from āsavas), (4) adhikāra
(service), and (5) chandatā (aspiration). Enlightenment of a disciple requires only two factors: (1) adhikāra and (2) chandatā. As regards the duration of time for their fulfilment of pāramīs, it takes two asaṅkhyeyyas and a hundred thousand aeons for the Enlightenment of a Paccekabuddha, one asaṅkhyeyya and a hundred thousand aeons for that of a Disciple. In realising the Four Truths, the former has no teacher while the latter has. For these reasons, are not the two kinds of Enlightenment basically different from each other?
The answer is that they cannot be different. For Ajātasattu will fulfil whatever is necessary for the attainment of Enlightenment as a Paccekabuddha, only after suffering for sixty thousand years in the Lohakumbhī hell. Indeed those who seek Enlightenment as a Disciple will gain it as Paccekabuddhas, if circumstances are not favourable for them to become Disciples. For they must have resolved to gain release as Paccekabuddhas. (This is the answer given by the first school of teachers. According to them, although the King had the potential for gaining release as a disciple, he could not do so in the present life because of his association with his evil friend, Devadatta, which made circumstances unfavourable and damaged the prospects for his attainment of sotāpatti-magga. But later on he will fulfil everything that will contribute to his attainment of Paccekabuddhahood and he will gain release.)
But according to other teachers, Ajātasattu had resolved to gain only the Enlightenment as a Paccekabuddha. But in the absence of any definite prediction of a Buddha, even those who have performed good deeds for Paccekabuddhahood cannot gain maturity of their Enlightenment in their capacity as Paccekabuddhas; instead they will attain Enlightenment as disciples in the presence of a Buddha. Hence the Buddha said: “Monks, if he had not put his father to death, he would have attained Sotāpatti Path while being seated here as he heard this Sāmaññā-phala Sutta.”
Of the three kinds of future personages, namely, the future Buddha, the future Paccekabuddha and the future Disciple, only the future Buddha is free from the pañcānantariya-kamma; the other two future Ones are not. That is true. Though Devadatta had been assured (though he had received the definite prediction) that he would become a Paccekabuddha, because of his grudge that he had long harboured, he committed the ānantariya-kamma by creating schism (saṅghabhedaka-kamma) and causing bloodshed to the Buddha (lohit’uppādaka-kamma) which were most serious crimes. Taking these into consideration, it may be understood that future Paccekabuddhas and future Disciples are not so invulnerable. It may also be understood therefore that King Ajātasattu missed his opportunity to gain Sotāpatti Knowledge in the present life because of his parricide and that he will later on become a Paccekabuddha by the name of Vijitāvī in accordance with the law of Paccekabuddha Enlightenment (Paccekabuddha-Bodhi Niyāma). This is the view of the other teachers. Choose between these two views what you think is more reasonable. (Exposition on the Sāmañña-phala Sutta, Sīlakkhandha Tika, Vol. II)"—Wisdom Library
I’m not sure I have any definite answers, but I’ve a few thoughts.
Hopefully, some of the very knowledgeable people in these parts may have better ideas!