Why do you equate CP with revenge?
You state quite categorically that "[it] serves NO purpose but revenge", but I haven't seen pro-CP advocates argue this. Other sentences are intended as punishments, not as acts of pure spite, so why is this different? CP is "simply" the severest punishment for the severest crimes.
Yes, we shouldn't be too cavalier about it, but we shouldn't be too emotional about it, either. We need to base arguments for both for and against on reason and principle, not emotion and whimsy. "It's just WRONG!" reminds me of the rhetoric of anti-abortion extremists.