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.