The death penalty is more philosophically ambiguous that most people think, and it leads to many people holding contradictory positions. For those against the death penalty, is it because you believe killing a person is always wrong, regardless of circumstances? Well, do you support euthanasia? Could you understand that some people may see capital punishment as a way to end the criminal's suffering under solitary confinement/life in prison, not unlike a person with terminal cancer? I admit that life in American prisons is much more excruciating than European prisons so this argument slightly changes across the pond, but let's stick to the problem of suffering. If you believe that life in prison is a more suitable punishment for the worst of criminals, is it because in this instance something akin to torture is justified? Why would you believe than that torturing someone like Khalid Sheik Mohammad was wrong? He was just as guilty as Dylan Roof.
The way I see it, and how I mitigate contradictory beliefs, I oppose how the death penalty is carried out in America - it's too commonly used on ordinary murderers, overwhelmingly black men who are frequently sorry for their crimes. It should be sparingly used on unrepentant criminals who committed senseless mass murders where the victims were picked randomly and the criminal's motives were completely callous. Personally, I don't care whether people like Dylan Roof are put to death or whither away in SC - both are extreme forms of punishment that unfortunately are here to stay in America, and choosing either one raises philosophical problems. I don't oppose the death penalty because I believe killing is always murder (my opposition is based on the possibility of innocence, the racial component in trials, cost in prison, etc.) - most people believe killing in self-defense is justifiable, and while I allow room for a right to death for terminally ill adults, it greatly conflicts me and I don't fall in a strong for or against position. Also, I believe torture is 100% wrong.
You can add why you justify your feelings about the death penalty, euthanasia, and torture/suffering/punishment make sense to you. I just thought some comments were interesting in how some feel he should be punished with the intent to suffer while believing it's worse to take his life, even someone like him.