In Of Mice and Men ? I believe there's no right decision to kill. Basically, he didn't want to responsibility anymore because he'd basically "always be a child" because of his disability. I don't think it's fair for him to take his life rather than let him move on. He felt like it was his responsibility to do something, he wanted to relieve his guilt - but in reality, I think he may have created more as time went on.
In the movie version (we had to watch it after reading the book in class) we all just sat there in disbelief, honestly. It's inhumane!
Good luck, rockstar! I hope all is well and you pass!