In the story Flowers For Algernon, why is Algernon's behavior a bad sign for Charlie?

Flowers for Algernon was originally a science fiction short story by Daniel Keyes. It got made into a movie called "Charlie" and Keyes expanded the short story to become a novel. 
The story is about Charlie Gordon who has an intellectual deficit (the wording in the story reflects standard wording of the era in which it was written, but that wording now is offensive to many, so this response will use language appropriate for the current era). He tries hard to learn and because of that he is offered an opportunity for an operation that would improve his IQ. One measure of his intellectual progress is the occasional race he has with a mouse named Algernon who has also had the operation. 
Charlie becomes really intelligent and becomes a researcher in the area of intelligence improvement. Because of this, he observes that Algernon is regressing and the mouse dies. Charlie dissects the mouse and sees that the mouse's brain reflects a loss of intelligence. 
The significance, then, of Algernon's behavior is that it causes Charlie to realize that his intelligence is also temporary. He realizes that he will regress, too, and will lose all the mental faculties he has enjoyed. 
The change in Algernon's behavior is the turn in what is essentially a tragedy. While the beginning of the story, which is told in the form of "Progress Reports" that Charlie writes, up to this point was about Charlie's improving intelligence and learning, after Algernon regresses, the Progress Reports show Charlie losing his ability to think. The irony is that only when Charlie begins to fail, he first laughs at an intellectually disabled man and then, in horror, realizes what he has done and he defends the man. 
Charlie's loss of intelligence also leads to changes at the factory in which he worked and then goes back to when he needs a job. Two men at the factory initially teased him and set him up; when Charlie returns to the factory, they defend him.