The death penalty was supposed to be for exceptional circumstances now, and look where we are. This country has put innocent people to death.
If you make exceptions, you will make more exceptions, and you are eventually guaranteed to put an innocent person to death due to the law of large numbers. A justice system must have a way to reverse mistakes to deliver justice properly, period.
Bad people doing bad things doesn't invalidate good people doing the right thing. If we kill innocent people that is bad, and we should require an incredibly high bar for this type of thing.
It becomes a slipper slope argument - well if we allow people to be jailed then inncocent people will be jailed and that's due to the law of large numbers, and there's no way to reverse our incredibly horrible prison system as it stands.
So now its my job to build our restorative justice system or... take out a few more nazis.