I am somewhat skeptical that free will exists. I think there may be some free will around the edges of minor things that don't matter much, but that most of a person's major decisions were made for him. Part of it is genetics, but part of it is upbringing, life experiences, psychological predispositions, and other things over which an individual has little control.
Believing that, it's easier for me to be forgiving of people who do even atrocious things. I see them as deeply bruised human beings with design defects. I suspect that in his heart of hearts, Hitler probably really did believe that Germany would be better off without all those Jews, and that Southern slave owners probably really did think that slavery was a part of the natural order, not to be messed with. Yet I still hold them morally accountable for the evil that they did.
How can this be? If Charles Manson is no more than an automaton guided by the passions of his nature, then how is it fair to punish him?
Because the fact that an individual may be hard wired toward certain beliefs or conduct doesn't mean that the hard wiring can't be changed. One of the things that comes from being human is the capacity to do self-assessments; to review one's life and ask probing questions. Not only is it possible; it is desirable. The older I get, the more I come to believe that the essence of ethics is the willingness to confront oneself with uncomfortable questions.
Having to live with the consequences of one's past is a powerful tool in causing that kind of self-analysis. If one learns that acting in certain ways will result in social ostracism, loss of economic opportunities, loss of important personal relationships, or, at the extreme, possibly even loss of liberty, a rational actor is going to at least ask the question if the behavior is worth it. If the answer is no, this can lead to changes in behavior. It can lead to changes in the behavior of other people who witness the consequences of that behavior.
I consider myself neither a true liberal nor a true conservative. One of the reasons I'm not a true liberal is my willingness to allow people to fail. If the consequences of success and the consequences of failure are the same, then there is no reason for anyone to ever ask if their lives are turning out the way they want, and to make any necessary changes. If Ted Bundy and Martin Luther King hold the same social esteem and enjoy the same prospects for a happy and prosperous life, then a major incentive to be good has disappeared.