I bet some of it probably has something to do with a person’s perception of the world they live in. “Life” or the world around us can seem pretty and full of hope but it can also seem ugly and full of despair. For someone like these brothers growing up amongst the things they did with an abusive mother, etc…..their perception of what the world is and what life is was probably distorted to begin with…. You could call him an animal and in some regards not be wrong. Their psychology is probably wired to be more primal and survivalistic by the nature of the environment they were raised in. The dude they executed, you might as well have caught a coyote and tried to question him about his actions and trying to develop trust.