Rehabilitation is only one of the criteria for taking away someone's liberty (or life, if rehabilitation is ruled out as a possibility).  The others are:
Deterrence - If you kill, you lose your life, just like when you walk off a 10th story balcony.  The laws of physics are absolute and self-enforcing.  Criminal laws lag woefully behind.  Why not kill someone you hate as a minor, if you know you're getting off light?  Few people are deterred by religious beliefs these days.
Retribution - If the justice system doesn't deliver proportional justice, parents, brothers or sons will.  Endless vendettas are not good for anybody.
Protection of society - Dangerous animals shouldn't be roaming the streets, even if they look like human beings.  They need to be caged or put to sleep.