I think that people are more likely to "suffer", if that's what people want, by having to live for the rest of their lives in a cell than if they are executed, although if it's taking years for that to happen, they're suffering stuck in the cell and not knowing when you're going to be executed.
I find it interesting that there are people who are against the death penalty except if it's someone you love, especially a child. That means you support the death penalty because what would make your pain so unique that you should get to know/watch someone die because of what they did to your family, but not someone else who had the same thing happen?
You either support the death penalty or you don't. You would either watch someone die or you wouldn't.
I don't support it, so no, I wouldn't accept it for anyone and while probably in tims past when this was normal I might have watched it once, I think I'd turn out to be like a lot of people who were sickened by it and watch once then never again. Hangings, etc were popular with some and unpopular with a lot more.
I don't think that we should only not support the death penalty because we might get the wrong person but we get the wrong person a lot and there are people awaiting execution in the US who are innocent but it will be pure luck and if they get taken up by groups which help those who are wrongly convicted which will decide their fate.
And there is no humane way of executing someone, although the guilotine is definitely the quickest and cleanest and would be my choice for me if I was being executed.