Eh, Carl was really cold about it, so needs an attitude adjustment, but I don't see the problem with what he did, either. The kid was shifty, and instead of putting the gun down, he was kinda inching towards the group while still holding it.
Yeah, not imminent danger, but definitely sketchy and not the behavior you should be taking in an active warzone at the end of the world. No consequences to killing you, so if someone tells you to drop it, effing drop it. I'm pretty firmly in the 'shoot him and don't take the chance' camp, given his behavior.
Carl just needs to be less of a dick about it. If he explained it like that, he's probably good. He insists on pretending the guy drew on him, makes him look bad when he doesn't need to, opening the door for Hershel's story to ring true.
"guy that came to kill us was coming towards me with the gun instead of dropping it, I was protecting the group and popped him", end of story.