I have a certain limit, where my conviction fades into a gray zone.
It comes down to:
Keeping an individual inside a locked space, for the duration of his life. This locked space is a waste, the food he eats, every day of his life - is a waste. Medical care and other attentions are a waste.
Mathematically speaking, it's all a huge waste.
We're always talking "worst of the worst" here, serial killers, spree shooters, child rapists/killers, and so on.
The cue should be this: Will we ever be able to release this person, into society, without him commiting similar crimes right away? If the answer is "nope, never." then it should come under consideration, of course along with an evaluation of, well, the "evilness" of a person.
Someone who believes he is Napoleon all his life, and therefore needs to be held in a room, fed, cared for, his entire life, did - after all - not end peoples lives, didn't tear apart some kids butthole, didn't ravage or ruin or destroy what can never be repaired or brought back.
Pain and loss must be taken into consideration.
However, in all of this I am also taking for granted a prison system that works as intended, one where mere druggies aren't gathered up, and where rehabilitation actually has precedence over simply punishing and stowing away.
To be absolutely fair I would consider - as a perfectly legitimate alternative to death, as the utmost penalty - to be banishment. In fact, in viking society, banishment was often used instead of death penalty, and it serves mostly the same purpose - you are left to your own device - see how long you make it, out in the woods, with just your fists and your teeth.