I don't think that anyone should call darkness what characters like Rick and Carol or anyone else go through, except for possibly Shane. He liked killing, I think. I agree, they have had dark times where it looks for all the world like they have hit rock bottom, but I think, if we have to label it, they are walking the "Do what you have to do to for the Group to survive" path and here, I believe Shane took it 'way too far. They have no evil intent - they have super-developed protective modes, but again, Shane was clear off the map. Nutjob. Megalomaniac. A monumental detriment to the Group. If you think that being the judge, jury, and executioner is wrong, then you need to remember that there aren't formal courts any longer and Walkers don't have the faintest clue about rule of law. Protecting The Group is all, but if any of the Group betrays the rest, they take what they get. If it's eye for an eye, so be it. If it's banishment, I'm not sure I would call that merciful, but certainly just. Most people just couldn't handle it and would be walker fodder in no time.
Morgan killing anyone he sees was during the time his wits were pretty much nonexistent. It doesn't make what he did right, but I think what he did was more a fact of life for him than it was about malice or justice. He had lost too much and the manner of his losing what he loved most tore his soul for awhile. I think it will hit home to him even more what his actions cost the group and he will change yet again if the guilt doesn't eat him alive (no pun intended). In my opinion, the Wolves are like mad dogs and if I were living in such a time, I'd put them down without a qualm. I think he will come to see that not everyone is capable of the change he made.