Well, he definitely did not break any rules. Therefore he does not deserve any punishment.
Also him saying he would kill a civilian in a video game is not something to worry about. Almost, if not all video gamers do that. Look at GTA. Nobody tries to follow real life laws in that game. If a couple of NPC's get in my way, surely I will run them down. I'm not a psychopath because I would never do such a thing in real life, and neither is he. However, if he were to do such a thing in real life, he still does not deserve his Steam account to be terminated. If he were a psycho killer in real life, Steam would not be the problem. Frankly, it's more likely that Steam would be helping him because it would be distracting him from killing real people. But even after saying all of this, he did say that he wouldn't do any of this in real life so there is nothing to worry about.
Also, if you want to report his account for hate speech or harassment, feel free, just unless there are a lot of people reporting him it's unlikely your report will have any effect on him. That's just how steam works. It doesn't worry unless it absolutely needs to.
~16