What about Good? By redeeming a villain, you give him/her a chance to atone for their crimes. If you can convince them to do so, they will help a lot more people than they would if you simply kill them. Is an abstract ideal worth depriving the world of kindness?What about their victims? What about justice? What about fairness???????????????????????????????????????????????????????????????????????
I am not suggesting that we let them go free without any punishment and any assurance that they will not continue on their previous path. But if you can truly redeem someone, don't you think their conscience will be more of a torture than any you can inflict? Also, you can set them working on correcting the things they've done that can be corrected (obviously, murder and the like cannot -- but they can reduce the suffering they've caused).