Wolfgang, I agree that "one's too many" has been used too often - but look at the situations you are applying it to -

"A kid has been mauled by a dog, one's too many. A woman has been raped by a former rapist out of jail, one's too many. An oil tanker spoils a coast, one's too many."

Dogs can be trained better, the rapist can be put into prison, the oil tankers can be redesigned. But with a human being, you can't say "ooops, better bring that one back to life".

I'm against the death penalty, not necessarily because of the criminal, but because of the victims. If someone you love has been killed, killing the killer won't bring them back. The killer needs to be punished, and kept away from society for a while, but killing them in return isn't right. The death penalty is just getting revenge, which I don't think is fair. (Quite apart from what it does to the judge, jury and executioner).
Crowds shout for the death of murderers, but if you give most of them a hatchet (or the switch for the electric chair), they will back away from killing them themselves. They just want it tidied away,quietly, with no mess, by other people. I think that's unfair too. (Would Ben Dover be willing to "shoot the buggers" himself, or does he just want them shot?)

I doubt I've managed to get my point across coherently, but I hope most of you understand.
Perhaps it could be summed up like this: If the whole world lived by "an eye for an eye and a tooth for a tooth...", we would all be blind and eating porridge. Blind revenge doesn't work.