At the end of the novel, George shoots Lennie. George does this to save Lennie from a more brutal death at the hands of Curley and his mob. It's a tragic end as George and Lennie had dreams of a future together on their own piece of land. This ending shows the harsh reality of their lives and how difficult it was for them to achieve their dreams in that unforgiving world.
At the end of the novel, George shoots Lennie. This is a tragic but almost inevitable event. Lennie had accidentally killed Curley's wife and George knew that if the others caught Lennie, he would be brutally killed. So, George made the heart - breaking decision to end Lennie's life himself, while talking about their dream of the farm to ease Lennie's passing.
It's a novella. The length and complexity of Of Mice and Men classify it as such. Novels tend to have a more extensive scope and development, which this story doesn't quite reach.