That's an individual who has gone out of their way to take the lives of multiple innocent people. Why does he get to keep what he took from others? Why do the families of the victims have to suffer him? It's not even a question of innocence.
If Manson was in any way sane, he'd realize that his actions were that of a monster. What is there to rehabilitate? If he's sane, how is living in a cell for decades not horrific punishment? A quick death would be a lot easier for everyone involved.
The death penalty is a harsh, horrible penalty. But everyone dies. I don't see how removing literal mass murderers is a detriment to society, unless we simply don't like the reality of it.
What's the alternative? Do we prefer situations like like Breivik?
In November 2012, Breivik wrote a 27-page letter of complaint to the prison authorities about the security restrictions he was being held under, claiming that the prison director personally wanted to punish him. Among his complaints were that his cell is not adequately heated and he has to wear three layers of clothing to stay warm, guards interfere with his strictly-planned daily schedule, his cell is poorly decorated and has no view, his reading lamp is inadequate, guards supervise him while he is brushing his teeth and shaving and put indirect mental pressure on him to finish quickly by tapping their feet while waiting, he is "not having candy" and he is served cold coffee, and he is strip-searched daily, sometimes by female guards. Authorities only lifted one minor restriction against Breivik; his rubber safety pen, which he described as an "almost indescribable manifestation of sadism," was replaced with an ordinary pen.[137]
In letters to foreign media outlets he told about his demands (in 2013) to prison authorities "including easier communication with the outside world and a PlayStation 3 to replace the current PlayStation 2, because it offers more suitable games"; media reported in 2014 about demands that he would starve himself to death if refused "access to a sofa and a bigger gym"; furthermore he said that "Other inmates have access to adult games while I only have the right to play less interesting kids’ games. One example is ‘Rayman Revolution,’ a game aimed at 3-year-olds," Breivik complained to prison officials."[138][139]