Of course he is completely responsible for his actions.  Kens point as I understand it is that many negative factors in this person's life helped to create the mindset capable of retaliating in this manner.  In the end an individual has to be held accountable for their individual choices, and he has been.  He's guilty of his crime, and others are guilty of stripping away his self worth.  Some people enter this world broken, others are broken needlessly.  It doesn't hurt a thing to acknowledge the impact our actions have on others or to place blame on cruel people for the roll their actions play in shaping the personalities of those around them.