If money were no object, I suppose I could live with locking themm in an oublette and forgetting about them. However, we are in a world where there are many priorities and not enough money to go around. If it costs more than $100,000 a year to keep a life/w/o/parole prisoner in the hoosegow (which is the estimate I've seen), that's a lot of money that should go to other things that benefit society. I'd rather put the money into programs designed to teach ghetto kids working skills and reading: there's at least a chance that some good will come of it. Money spent to confine a lifer is just a sunk cost.
Understand, I see this as a practical approach, not perhaps the most morally ideal approach. I'm just not willing to pay higher taxes to support some murderer's comfortable (or even his or her uncomfortable) life in jail that could last 30-40 years or more.
Call it conceit, or what have you, but I like to think that humans are something more than animals.