I think Dennis Miller put it best... "yes, the death penalty is - arguably - barbaric and uncivilized... but sometimes, you just have the thin the herd..." (paraphrasing, but that's the gist)...
..meaning, no matter how ugly it is, sometimes it's the only way to truly effectively deal with the problem, to give the victim's family any kind of closure, and protect the public at large.
I don't like the DP, but if someone murdered my wife and/or kids, I can't see myself losing any sleep over their killer's execution.