If someone kills a bunch of people no amount of philosophical quibbling and defining is going to make me think that person should be allowed to continue living in society, justice simply couldn’t be a concept at all in the absence of some form of free will, yet we require justice to cooperate in making better lives for ourselves. So the value of acting as if we have free will is more valuable than an esoteric philosophical truth.

Free will or not - if you have intentionally killed a bunch of people in the past it's to be expected you're likely to do it again. Such person shouldn't be put to jail because we want to punish him. After all he could not have done otherwise. However as they're danger to others something clearly needs to be done. They have to be separated from society in some way to prevent further harm but we should still treat them humanely and make sure their live is as good as it could be withing the circumstances.

If a bear wanders onto residential area we don't shoot it because it's evil. In my opinion the bear is no different from a murdered. They're both slaves of their biology.