Now, I don’t know what the good venerables are saying in the videos above, but from my own study of the suttas I have gained a view of brahmavihara which I think differ a bit from the common Visuddhimagga based practice.
First, almost everytime a lay person is releasing his heart by metta, karuna, mudita and upekkha in the suttas, the first thing he does to develop those states is that he abandons the ten unwholesome actions and is mindful of the ten wholesome actions. When he scrutinize himself and see that he is totaly free from the unwholesome actions, that he has abandoned them, he feels very happy and this starts a process that leads to stillness of his mind. Based on this stillness he releases his heart by metta, karuna, mudita and upekkha.
Now, there is a thing about metta which you should have in mind to understand this process, and that is the fact that metta isn’t “loving kindness” in the sense of an active outward loving of other creatures. It’s more of a withdrawal from the world whith no interest in harming anyone, nor by body, speech or mind. This state of mind is something you develop by establishing right view.
If you tale a look at the ten unwholesome actions, the first one is the most gross unwholesome deed, killing other beeings. If you kill other beeings you are not practicing at all. So you abandon that. Then you abandon stealing, abusive sexuality, lying, and so on. You work through all the ten actions which gets more and more sublime and difficult to abandon, until you finally abandon unwholesome mental actions - greed, ill will and wrong view.
Wrong view is basically that your life and actions are destroyed with death. Lord Buddha says many times that moral actions are based on right view, even though you think that you can be moral without it. The thing is that when you practice to establish right view of life and death, that you are reborn, you begin to restrain yourself because you don’t want to fall down to hell. You want to incline to heaven.
So basically, you practice to establish right view and then step by step, based on this view, you abandon the unwholesome actions until you purify your self from the evil mental states of greed and ill will. Now you reach the last step, right view, which becomes fulfilled and established.
In Pataliya sutta (Madhyama Agama 20) [pali parallel SN 42.13] a man is abandoning all the unwholesome deeds, and when he review himself he sees that they are abandoned, and then he is filled with joy, attains stillness of mind and realeases metta, mudita, karuna and upekkha. In this state he thinks that “Now I don’t care if there comes a heaven or not after death, because I have attained stillness of mind”. This is attaining of view, opening the eye of Dhamma.
Therefore, you must focus to abandon the ten unwholesome actions and be mindful of the ten wholesome actions to develop metta. Metta is the intention not to harm anyone, it is a state of mind where you look at the beings in universe and think “I don’t want to harm anyone, my mind is stilled”.