People that have been indoctrinated into cults, which is basically what we're talking about here, can be successfully deprogrammed.
However, that doesn't mean they should be rehabilitated - ie allowed back into society, for a couple of reasons.
Firstly, you can never really know whether the deprogramming is successful. The risk of relapse is always there, and not worth taking.
Secondly, they don't deserve the chance. Some crimes are so great - ie random mass murder - that they should result in exclusion from society forever.
I don't buy this concept that you have to be mentally ill to commit an atrocity. I think you can also be just a bad person, and/or deeply misguided.