"This is the only reason I'm against the death penalty in practice."
Obviously, it isn't the only one for me or the legal system of all western countries except the USA.
"And therefore I have no problem with it in this case."
Have you even read that Breivik wants this to happen? He didn't make it this obvious and this terrible without some horrible ideas behind it.
"And what do you do if they're not mentally ill, but simply evil? Some people just enjoy killing, without having anything wrong with their brain, just with their soul. (This is an honest question, BTW.)"
Can you prove that someone is simply evil? Because I'm pretty sure that's impossible. The soul is a nonlegal entity, you can't use it as "proof". Evil for evils sake mostly happens in cartoons for children. Oh and at least here in Germany, people who are thought of as too dangerous for society by multiple experts are put under protective custody whether they are mentally ill or not.
"As far as I'm concerned, vengeance is a necessary part of justice. People should be punished not just as a way to protect society, or to "rehabilitate" them. I believe that punishing evildoers is a worthy goal in and of itself."
"Much like the death penalty, the only reason I feel the government shouldn't torture is because they could do it to someone innocent. But Breivik isn't. There is absolutely no doubt as to his guilt. Survivors of the massacre, and family members of his victims, should be offered the chance to have a go at him."
Yeah, answer violence with even more violence. Aside from the dubious "emotional relief" that would give them, it would also lower them, not on his level, but still downwards. It might be satisfying short-term, but the long-term consequences, emotional, social and ethical, are nothing to be proud of.
I don't know what I can say to you about that other than that it absolutely is NOT part of most western countries justice system (not sure about the US though). As I've said before, "being evil" is not something you can proof. And even criminals have rights. Human rights to be exact. Which as I've stated before are worthless if we just declare them "not counting for evil-doers". I know that the first impulse when it comes to punishing people is to enact vengeance upon them, often violently. Hell, I'll often get the urge to punch such people myself. But the first emotional impulse shouldn't dictate the way we act. It has been shown time and time again that this just gives lynch mobs, revenge cycles, terrorism etc. fertile ground to grow on. Terrorists like Breivik want such "violent" societies, because they think that it is the only way to "cleanse" their country of everyone standing against them. Just think about it: What if someone declares homosexuals or unbelievers to be evil and worthy to enact vengeance upon? Which is happening in quite a few countries by the way. Look at Dutertes Phillipines in which thousands (including innocents) are being killed in the name of a supposed "war against drugs" by killer squads. It can't happen nearly as fast (or possibly at all, at least not governmentally approved) in a country in which human rights, including the one to life and physical inviolability are the law for everyone.
Believe me, I can understand you, but you would do Breivik a favor by executing him. By locking him up, ignoring his damn creeds, giving him the humane treatment he wants to withhold from others we are showing him that we as a society are better than him and his ideology. This punishment in my opinion is MUCH more satisfying and helpful in the long run, even though it seems to be counterintuitive at first. Considering his moaning and bitching about his sentence, I believe he himself knows that too.