After Siddiq's death and Dante's betrayal, Aaron and everyone else in Alexandria were especially wary and distrustful of outsiders. Especially when these "Whisperers" were swarming everywhere, caution no longer hurt anyone. No one wanted to accept newcomers into their community, and this was very understandable. They had been betrayed so many times that you began to trust no one. This happened to Aaron. Moreover, when he also had a daughter now, he became even more skeptical. After Siddiq's funeral, he wanted to get some fresh air, and so he went into the thick of the forest to kill a couple of walkers, but accidentally saved one guy.