Why didn't you tell me that this would happen to me or even Algernon ?
Charlie goes to work the next day and realizes there's a petition to get him fired. So he quits.
He then moves to New York with Algernon. He notices Algernon's behavior is changing, so he starts to study on his behavior and finds out something interesting.
After investigating Algernon's behavior he learns that Algernon's behavior is coming from the procedure he's been in, which is the same procedure Charlie took to get smarter. Charlie fears the same thing will happen to him.
Algernon dies and Charlie decides he doesn't want to be apart of Dr. Nemur and Dr. Strauss's operation anymore. He sends them a letter explaining what he has learned about what Algernon went through, how the experiment caused it and how he thinks it will happen to him next.