Just for the sake of argument. You say that because he killed and probably will kill again, death penalty is justifiable . By the same line of reasoning this should be valid as well for the judge, the attorney and every other person responsible for the final execution. You could even make the argument for the victim, as he killed people by actively rejecting proper medical care in multiple cases.
Still a not a valid argument. It is an argument of authority, a typical logical fallacy. Just because a group of people is reputable and says it right does not mean it is right. I mean a possible jail sentence would be as adequate to prevent him from doing it again, as a counter example. So the argument for killing him would basically narrow down to “because authorities said it is okay”.