Why did George kill Lennie in the book Of Mice and Men?

The ultimate reason why George killed Lennie is because George knew that it was more humane to end Lennie's life quickly, the police or others like Curley who wanted revenge not justice might not be as merciful and quick so George did what he thought was right.

George loved Lennie, but he knew that, although Lennie had a tender and innocent manner, he could be violent. George new that Lennie could get uncontrollable, somewhat like a wild animal.

He murdered a girl and even though Lennie didn't understand what he did, he would have to answer for her death it isn't like the mice or the puppy that could just be replaced. George just couldn't turn Lennie over to the police, he knew that Lennie wouldn't be able to handle being separated from him. Even though Lennie was slower than George, he still had a big heart.

Also it was a symbolism of George killing his dream of owning a ranch and it was linked to the way Candy had let his dog be shot when it was doing more bad than good. Candy said he should have shot the dog himself and George remembered this and new he had to do it himself. It was a more humane way of killing Lennie than him being lynched, George recognized this and ended Lennie's life with respect and dignity.