RE: Zero Quantum
He can spend life for that, just like us. If he has Disin that could be a problem.
RE: Mentally Dominated
Iron Will counters.
RE: Judgement
It's more like that he doesn't trust Supergirl's.
We are dealing with a rabid dog who in Steve's view *can't* know anything useful, much less anything worth the risk of keeping him around. We have several people on this team who will argue against killing him... presumably leaving him to kill and eat other people. Cody wants to show off and/or excersize his curiosity... neither of which is especially useful in this situation.
In Steve's judgment this is someone who should be killed at first opportunity, which will be in a few seconds. And we're in a combat situation where we don't have the ability or time to discuss the matter.
RE: So...basically Steve no longer trusts Cody's judgment
What? You mean you don't trust Steve's judgment?

He's supposed to be the older, wiser head... (joke).
More seriously; Since Steve already knows the guy doesn't know what you want to ask him about he doesn't see the point. We're going to have to shoot this guy at some point so best to do it now.