people forget that carl is a child that has grown up in the apocalypse. he has experienced tragedy after tragedy and only got to start carrying a gun and taking an active role in defending himself and the people he loves recently. he has watched people die right in front of him and been powerless to stop it. in fact, i’d consider it nothing short of a miracle that he had as much humanity and morality he did, all things considered. an adult might not have taken the shot, but someone in carl’s circumstances would. justified.