Was there ambiguity about what Carl did to that teenager? Sure. Can you reason it out both ways? Absolutely. People are missing the point though. It's not Carl shooting the teenager that is the problem. It's what's going on in his head that's the problem. What he said to his dad should be a red flag.
He is thinking in absolutes. He claimed that Rick should have taken the chance to kill the Governor when they met the 1st time. That could have likely ended up with Daryl, Hershel & Rick himself getting killed.
Despite whether you think Carl was justified in this one situation, his motives are questionable. He will be an executioner far more than he will be a group helper. Hell, they were at such close range, with the advantage, he could have just put a bullet in the kid's hand or arm to make him drop the weapon, but he jumped straight to execution, & whether they were at risk from the teen is not the point.
Why Carl has become so absolute about putting people down is the point, & he doesn't seem to wrestle at all with the morality of that, the way Daryl or Rick do. Righteous men wrestle with that
There will always be risks. Life is risk. Assuming execution is the perfect solution to a risky situation in a dangerous world is reckless & potentially morally corrupting. I wouldn't go so far as to say we're there yet, but it's looming on the horizon, because more than anything... Right now, Carl's out to prove he is hard
And well... that's an extremely shitty motivation, & if it continues, it will cause him more pain than relief, I assure you.
Rick has cause to be concerned, whether this one execution tells the tale or not