Charlie returns to New York because he is frustrated and little overwhelmed by the awarenesses and understandings effected by his increased intelligence. As his IQ rises, he recovers lost memories from his childhood, and he finds that his growing relationship with Alice is adversely affected by his traumatic past. Also, since he is now functioning at a genius level, he finds it is hard to communicate with other people.
Charlie's view of Nemur and Strauss, the psychologist and neurologist who engineered the experiment that gave him his new intelligence, has changed as well. He is insulted at being treated like an exhibition instead of a person, and is appalled at errors he now sees occurred in the handling of his operation. Angry and disgusted, he retires to New York, where he lives a life of increasing isolation.
Charlie doesn't move to New York. He moves away from it at the very end, after his intelligence has disintegrated. "Thats why Im going away from New York for good. I dont want to do nothing like that agen."
He's definitely been changed by his experience as a genius, but I don't see anger or hostility at all. His journals have been pretty honest throughout the whole tale, starting because of his ignorance, and continuing because he wants to remember after Algernon regresses. One of the main differences in Charlie at the end is that he now knows when people are making fun of him, and no one would call that pleasant.
Charlie's determined spirit that got him chosen for the experiment shows at the end, and while I do see sadness, I don't see anger. He says he's grateful that he got to experience everything that he did, and he blames himself for losing it because maybe he "dint try hard enuff." He sends messages to the people he cares about in his journal, and even tells Miss Kinnian not to feel sorry for him. He recognizes that maybe he did something special "for all the dumb pepul like me." He says he's going to take books and try to read and be smart, even if he doesn't remember. He sends advice to Dr. Nemur on how to make more friends.
Charlie just knows he can't go back to being a genus, so he wants to go where no one knows he "was once a genius and now he cant even reed a book or rite good."