Riiiiight. You are saying that in order to protect other inmates he should serve his sentence and then be released; where he will have the freedom to rape and murder innocent women and children instead?
Prisons have religious staff, education staff, guards, volunteers, other vulnerable prisoners, legal visitors and others. I have been in many prisons many times and never been convicted of a crime.
I'm saying that with decades inside and a life licence he will be supervised and able to be recalled at any point. He will have to abide by strict conditions and if the system does its job, won't offend again.
Now the system doesn't always do its job. So proper supervision, well-funded services and supervision and other things need to be in place. THAT'S where I'd put my energy into ensuring.