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.
However, the writers are brilliant enough to give Chandler Riggs, the actor who plays Carl, dialogue that would be consistent with how a 11-12 year old woulld actually react in such a situation - which is a partial truth about what happened.
As an aside, it's interesting to read what Riggs has to say on Twitter
, responding to questions about what he did for Easter, when they start filming again, and getting ready tor High School next year.