I prefer life imprisonment over the death penalty TBH.
First because of the possibility of miscarriages of justice: how many have been condemned to death, only to have their name cleared only after
their execution? You can still release someone who's done time for nothing, you can't bring back to life someone who's been wrongfully executed.
But even if you only consider those who are obviously guilty, like the guy you're talking about, killing the bastard won't bring his victims back to life either anyway. Let him rot in captivity for the rest of his miserable life instead, that's all he deserves — besides, given how long the waiting time before being executed can get (and how some death row inmates often die of natural causes before that), you have there yet another reason that makes death penalty pretty pointless IMHO.