To a certain extent, yeah, that's not how it works, really. Like, even on a show as violent as Supernatural, the boys don't kill humans, even if they kinda deserve it. (I don't want to spoil you for later seasons, here, so I won't go into any more depth on this point and the ramifications of who killed who in various situations.) Having a once-off character kill a human lets the human get killed in a less lingering way.

Also, of course you got the Tevriels jumping in on this one. Who else do you know who fangirls both Supernatural and Captain Jack Torchwood as hard as we do? ;)