To say that violence is driven by anger, and that anger is rooted in fear, is not to mitigate the culpability of a frightened and angry murderer or any other criminal that engages in violence after experiencing fear and rage. On the contrary, we all need to make rational choices in our lives regardless of our emotional state, and we are all legally responsible for our actions.
However, the law notwithstanding, my experience as a criminologist has shown me that there are times when human emotions trump rationality, and no emotion is more powerful or motivating than fear. If you doubt this conclusion, ask a violent person to look beneath his anger and describe his feelings. If he is honest, he will describe his fear.