THATS the point of the show. The Line between right and wrong, murder and killing, is survival alone is enough. The self awareness of a group.
You can easily imagine without a counter balance, Rick's group, down the line, can become no difference from the group of Humans they've faced. This happens in the real world ALL THE TIME, thats what this show/book explores.
Carol already crossed that line, she slaughtered the people in the prison, with ALL good attentions, cuz she predicted a danger to the group's survival, and ignored humanity. Should Rick have killed her when he found out? Should Rick have trusted that she would've come back and killed him? The "humane" decisions have been the most important decisions taken on this show.
To say "humanity" doesnt matter anymore is the reaction that always gets me, and exactly why this show is still interesting to me. Without the Morgans and Hershs, Rick and Carol can very likely slip into darker and darker actions.