People who keep saying his transformation is too much have seriously never felt true despair and rage.
Every person can become like this, just takes the right ammount of bad things in the correct order to make them despair and for that despair to tapp into the buried rage of a past violent experience.
He is hitting rock bottom, no hope, no friends to help, only thing he has in his mind is vengance even if it costs him everything.
Good news is he can only go up from there.