Taxi Driver

Travis' character develops throughout Taxi Driver and we can see a turning point during the movie. This occurs after Betsy basically tells Travis that she hates him, and his dreams of loving and being with her are completely crushed. This literally changes his outlook on life and turns him into a hateful person. It is also after Travis sees Iris in the street after almost running her over. I think he realized after he saw Matthew abuse her and pull her out of the taxi that he wanted to protect her. This explains why he buys all the guns, gets into shape, and tries to improve himself. He goes from being a normal taxi driver who is pleasant and charming to this serious, lying man whose one goal is to kill. Even though at this point in the movie (picture below), we do not know who he wants to attack, we can tell that it is going to be more than one person.
I knew that from the incident where Iris gets into Travis/ taxi, she was going to be important in the movie and it obviously made an impact on Travis. He became really shady and went out to find Iris and take her with him, to help her escape from the nightmare that is her daily life as a prostitute. In a way, Travis finds a new sense of love after meeting Iris, but more like a father-daughter kind of love. It would add another level of creepiness to this already sketchy film if they had something more than this kind of relationship. Travis goes from loving Betsy, to hating the world, to loving Iris.

Was there anything else that changed Travis' outlook on life?