Rehabilitation of violent criminals is a farce, drummed up 40 years ago by people who should be old enough today to admit that they failed to grasp reality.
Violent predators aren't going to mellow out and become normal citizens.
They can however put on a convincing act while in prison, where they pretend to be remorseful for their crimes in order to win the support of the parole board.
Most criminals, even violent one are not predators. Violent predators are indeed not usually going to become normal citizens, that is what long term stay mental jails/closed facilities are for.
There is a chance for loads of criminals, even violent ones to get and stay out of jail, but then you have to do something to rehabilitate and not create monsters out of them like is done in a lot of prisons. There are notable exceptions though, predators is one you mentioned, gang members are another, psychiatric patients is a third group and drug addicts are the third group. Most others IMO do have a chance at rehabilitation, even violent ones.
Predators are indeed such a danger that normal prison sentences do not solve that issue. For that reason in the Netherlands we have TBS, which is purely for a class of people who are deemed "virtually incurable" of their dangerous criminal/violent tendencies. Those people usually get a shorter stay in prison than regular criminals do, but when their prison term is over they go to a closed facility where they will be treated until there is a reasonably high certainty that person will not fall back into their criminal behavior, or they give up the treatment and lock people up until they die.
There is one case in which a man had dangerous child predatory instincts that could have lead him to commit rapes and murders, he was arrested on after a serious sexual incident with an 11 year old. Psychiatric evaluation found him as said ot have dangerous predatory instincts with children. A normal rapist, even of a child would have been released already, this guy however if seen as incurable and has been in TBS for 56 years and will stay in long stay care until he dies. The only thing that has changed over the past 56 years, is that he no longer gets forcible treatment anymore (he won a lawsuit 5 years ago to achieve that).
In this system of TBS, a person that should have gotten 12 years of jail time now has been jailed for 56 years and will remain locked up and not a danger until he dies. Just because he is a predator he gets indefinite incarceration and forced treatment/medication.
That is how you keep predators out of society and out of general population in jails IMHO.