Why is it so important that George opens up to Sli
Since the story end with George killing Lennie, we need to understand the story of George and Lennie. Like why George travels with Lennie and why he looks after and protects him, and we don't just think George was horrible to Lennie and shot him just because he could. And so we can also see the softer side of George, instead of the character we've seen at the beggining who is seen as bossy and unreasonable towards Lennie.
What died along with Curley's wife?
The dream of George and Lennie's. (And Candy's). When Lennie Kill's Curley's wife, George immediatly knows that the dream can never happen. Even though George tells Lennie about their dream all of the way through, getting so close to the dream coming true (Finding the ranch and gathering the money for it) he started to believe that it could really happen.
Why is it important that Candy's dog was shot?
Candy's dog was shot because it was old and was no help to anyone anymore. And we know that the dog was shot in the back of the head to immediately kill the dog with less pain. So at the end when George has to kill Lennie, Lennie is shot in the same way.
Wh did George kill Lennie?
When Curley finds his wife dead with a broken neck, he immediately knows that it was Lennie. And Curley sets out with his men to get Lennie and eventually kill him. George knows that Curley finding Lennie first would result in Lennie getting beaten and tortured, but if he found him first, he could make sure he didn't suffer. So he ends his life in a much quicker, painless way than it would've been with Curley.