Why did George shoot Lennie?

At the end of the book, George decides to shoot Lennie. I think he does this for many reasons. I think the main reason is that Lennie isn't bright. George said, "Lennie never done it in meanness. All the time he done bad things, but he never done one of 'em mean" (95). Lennie kept making bad mistakes that put him and George in danger. After Lennie killed Curley's wife, George knew that there would be no more mistakes. Also, Curley was going to kill Lennie himself after he found out that Lennie killed Curley's wife. Curley was raged and he was going to kill Lennie. Also, I think Lennie was a huge liabilty to George all of the time. Lennie doesn't understand things. Lennie makes bad mistakes. Plus he has lots of breakdowns over simple things like ketchup for beans. I think George killed Lennie because of his mental insanity and Curley was also going to kill him.

What do you think about why George killed Lennie?