I think Carl's actions were explainable. The kid appeared to be turning the gun towards him and seemed to be going to try something. The "come get it" thing - he could have possibly overpowered Carl.
That being said I can see where Herschel saw it differently as well. The thing is Rick is going to be dealing with Herschel's account of it. Carl did not say it was wrong he said he did what he had to do and basically told Rick off for not taking the Governor out when he had the chance. Rick is concerned that humanity is going away and that Carl is going to be about pure survival. I actually thought it was an interesting twist.
"Do not overcome by the evil, but overcome, in the good, the evil" - Romans 12:21 - Young's Literal Translation
"Use peaceful means where they are appropriate; but where they are not appropriate, do not hesitate to resort to more forceful - Thupten Gyatso (the Dalai Lama, 1932)
"It is not the will to win that matters - everyone has that. It's the will to prepare to win that matters" Coach Paul 'Bear' Bryant.
"The ink of scholars is more precious than the blood of martyrs" - Muhammed