Some people can not be rehabilitated, period, barring massive brain surgery that we are not yet capable of. Psychopathy is lack of blood flow to certain parts of the brain that process emotion.

I am friends with a psychopath of the highest degree and there is no way to change someone like that. He literally can not feel emotions, though he tries to understand them. When he writes, though he is a fabulous writer, his characters feel flat because he can not process the emotion behind them. This isn't a case of someone with a complex. A psychopath literally has pieces missing that can not be put back. He, at least, has decided to live his life as normally as he can without indulging in his darker sides very often. But someone who has already murdered...

Have you ever heard of Ed Kemper? Or perhaps Karl Panzram? There are people that are not rehabilitatable and it is naive to think that's simply because we aren't "trying hard enough."