When he learned that the regression would come, why did Charlie decide to return to work at the institute?

Charlie's situation at this point in the story is tragic. The experiment that gave him superior intelligence has allowed him to discover the Algernon-Gordon effect, which predicts he will continue to regress. As his intelligence wanes, he is faced not only with the emotional fallout this creates, but also the reality that he has no job and no money, having quit his job at the laboratory. Mrs. Flynn, his landlady, despite her concern for him, needs the rent. And so Charlie, out of intelligence and options, goes back to the only life he knows--that of janitor at the factory with his "friends."