Lennie found a mouse and decides to keep and pet it. However he doesn't realize how strong he is. He pets it and kills it! He likes to pet and feel animals and anything with a texture. Although he can't control how hard hes touching them. In the end this reveals the struggle George is going to have with Lennie. Also showing how he has a disablilty and can't control his impulses and strength.
Carlson ends Candy's old dogs life by shooting him in the back of the head. Meanwhile this was very hard for Candy to allow happen. However the dog was very old and useless. For example everyone knew it was the best decision. There was no purpose to his life anymore. The theme of this is that once things are useless they have no reason to live. Also that their a waste of space and it's in their best interest to end it.
Lennie killed the dog and Curly's wife in the barn. He broke both of their necks by accident. He just wanted to pet and feel their hair for longer. For example Lennie was feeling her hair and she freaked out and he panicked and broke her next. He didn't mean to.
Lennie got himself in a lot of trouble and hid in the spot George told him too. Then unfortunately George shot him in the back of the head. He did this because it was in Lennie's best interest. It was better that he got shot by someone who loved him,George, rather then whatever the other people were going to do to them once they found him. Overall not all dreams come true. George and Lennie's dream and when Lennie died so did the dream. Which supports the theme.