Mrs Edmund's death. This woman was nothing but sweet and kind to Alex, even in the midst of the apocalypse, acting almost as a second mother to him. And what does she get out of it? She gets raped and killed by Ferret and Target respectively. What makes it even worse is that Alex feels responsible, because of the possibility of them following him, even though it's just as likely that they simply saw a defenseless middle aged woman in her home and decided to victimize her.
Darla’s reaction to the above counts as well. After initially being angry at Alex, she retreats inwards, not speaking for the next few days, and clinging to her past rabbit, and Alex for support.
