Mentally insane

So, he's nuts! My questions are:
He violently killed people close to him. Is he at all likely to kill again? Yes.
Is there a method to get him to not kill, apart from executing him? Maybe.
Which makes society safer execution or imprissonment with possibility of parole or escape? Execution.
Well, this non-English speaking murderer, I think just about spelled out his end with morte, no comprende

That's a damned shame that he's crazy. He's not going to get sane nor nonviolent, so ... we're keeping him around, why?