Yeah but consider that he has Terminus fresh on his mind. People offering peace and luring you in just to enslave and eat you. As he said in this last episode, people are the real threat now. I dont disagree with you that it was cowardly, but I believe it was still justifiable until he knows who he is dealing with. Say he started beating him senseless or actually started the action to kill Aaron if the group didnt come back on time just for Aaron to say that their plan was to entrap Rick's group, and Aaron divulge this info to spare his own life. Although unlikely, they are still heightened threats to Rick. Especially in their given situation.
As for humanity, while it needs to be preserved, they have been on the run for years. Look at the stress troops take on 8mo deployments. Now imagine that as never getting a LOA and always staying on the defensive. For 3 years they have been in survival mode. Not knowing if they will live to see the next day, not knowing where the next meal will come from or when they will eat. Psychologically they have issues, as seen at Alexandria and the dinner party. They have a long way to make it back to humanity. I think this is the big contrast we are seeing now between the two groups.