There is no one so cartoonishly evil that it’s unsafe to even keep them in prison.
Most of us have not seen true evil up close. Most of us cannot even imagine it. We just believe we can, absolutely certain in the arrogance of our own ignorance that we already know everything we would ever need to know about it.
But in reality, we look away from even the brief glimpses of evil we do see, choosing instead some variation on the just world theory to make it sit better. An example of this might be finding a way to blame a victim of bullying or abuse for some level of unwise personal choice that makes it mostly their own fault -- in our own sight -- and thus keeps us from having to deal directly with the wrongdoer in any way.
That's just a very mild example, but it's all around us. We humans don't want to see evil. It's a direct threat to our innate sense of security and stability -- and why stochastic terrorism is on the to-do list of every budding autocrat. We "missing stair" evildoers ALL the goddamn time, in every culture, in every country, law enforcement and laymen alike. Most of us are not even equipped to look at evil, much less determine its true nature or potential for harm to others.
And that's kind of where I clock this opinion: well meaning, hits the right virtue buttons, but basically uninformed in terms of personal experience.
I won't go into what I have seen personally. I'll just say I genuinely wish there were no such thing as "so cartoonishly evil that it’s unsafe to even keep them in prison." You're very privileged in so many ways just to be able to think that, and I hope you never understand just how lucky you are in that regard.