1. Meihecha teisi he won’t? Why assume he’ll do TESHUVA?
2. We’re not. Ba’asher hu shom, he’s a rotzeiach rasha who would kill a Yid in a second if given the opportunity.
3. Known terrorists
Would you say the same for a Jew? And careful, because there have been.
He should be incarcerated until they are certain he will not act again, and if that is never, then so be it. Just because the Medinah may release people before then doesn’t mean that a Yochid should kill someone.