Being found guilty of serial murder implies neither actual guilt nor, if actually guilty, that the murderer views other humans as playthings. A murderer may, in fact, believe they themselves are removing such people from the world.

It’s probably also better if potential jurors don’t have the thought, “Well, I have some doubt but if this guy dies and I’m wrong, at least it’ll do a lot of good.”

The parent comment said "known serial killer". There was no discussion of guilt. I may be wrong about the play thing bit so to revise and be more specific in my stance;

1. If they have no issue with killing people.

2. have killed people

3. will continue to kill people

If all or even most of those victims being people who do not harm others then I'm alright with the murderers being used in experiments or killed themselves.

I don't care about them being found guilty or not, this is hypothetical and there are a number of ways that they could be found guilty when they haven't done it or not guilty when they have. Getting to the point where you can know without a single doubt that they in fact are serial killers is a different matter.

I did not get the sense @newsbinator was trying to remove the question to an entirely hypothetical realm, but stipulating that we somehow have this knowledge with hypothetical 100% certainty is fair enough.

(1) is unknowable. (2) and (3) describe many, many people that are not generally considered to be serial killers (e.g., a soldier). If a 'known serial killer' were experimented on and died, (2) and (3) could also describe the researchers doing the experimentation.

We could modify (1) to say, "...and we have 100% certainty of this." But I guess really not certain what value the question has if we basically require 100% certain knowledge of the events and the internal state of the perpetrator. A 'no' answer would mean 'no' in the real world, too, but a 'yes' answer doesn't really tell us anything about the real world.

For the sake of simplicity, I used "known serial killer" to mean somebody we are certain is a serial killer for whatever reason. Let's say they were at a parade, in front of 100 eye-witnesses, and live tv cameras, going around killing people, and then confessed to it to arresting officers and in court.

Somebody who is doubtlessly (and perhaps unrealistically) guilty of committing one of our highest crimes, for the sake of simplifying the question.