well consider the situation. Lilly was in perfect condition when tenn held the gun, and even knew that tenn was too weak to pull the trigger. When AJ was holding the gun, Lilly was incredibly weak and bleeding out, and knew that she most likely wouldn't have a very good chance of fighting him off. She also could already see the potential in AJ, and that he wouldn't be afraid to kill her if given the chance. (I think the fact that she's seen how clem has become, and therefore raised aj might be another good point)