Prison is infinitely more cruel than a quick and dirty execution.
Want a good comparison? Put a cloth band-aid on the hairiest part of your body. Press it down good'n'hard. Now would you rather rip it off fast and only have it take out a few hairs, or excruciatingly slow and feel each hair pulled to the limit before ripping out of it's lil growin' place?
Prison is a breeding ground for monsters, why throw another one into the mix, eh?
That's a pretty weak comparison. I won't bother to explain why.
So is the argument now that those convicted of murder (or rape or whatever is on the death penalty list lately) be killed FOR THEIR OWN SAKE, to avoid being cruel by NOT killing them? Silly.
As for excruciating, imagine sitting in a cell knowing that on some future date the government will strap you in and take your life. Considerably worse than knowing you'll just die of old age. Plenty of geriatrics feel mentally or physically imprisoned in their final years anyway...
And no, I wouldn't go along with any suggestion that the execution process be sped up. There have already been too many death row and life-imprisonment prisoners exonerated as is. Reducing their (or those on the outside working to determine the truth) chances further doesn't strike me as moving in the direction of greater justice.
And I've probably said it before, but I'll say it again, murderers and rapists are not "monsters" they are human beings. Deeply flawed and fallen human beings, true, but still human. Labelling them as monsters seems rather childish to me.