At the end, Lennie is shot by George. This is because Lennie has caused trouble and George knows it's the only way to protect him from a more cruel punishment.
It could be due to his mental state and the stress of the events. Lennie's mind was already fragile, and the culmination of everything might have pushed him over the edge, causing hallucinations.
Lennie's character transformation in the novel is quite notable. At first, he's just a simple-minded guy with good intentions. However, events lead him to make mistakes that have tragic outcomes, showing a darker side to his nature.
The main characters are without a doubt George and Lennie. George is the more practical and cautious one. He plans for their future and tries to make sure they stay on track. Lennie is full of child - like innocence. His actions are often driven by his immediate desires, like touching soft things. In fan fiction, their relationship is usually explored further, showing how they interact with the world around them, with George constantly guiding Lennie through various situations.