In the book Of Mice and Men the author shows that George mercy Killing of Lenny is the right thing to do. For example when they kill Candy's dog, and Slim approved what Candy's said about he should have shot the dog his self. Is better that someone that knows him and really fell bad about It kills him, that someone that doesn't care about him. George fell badly about It, but he prefers to kill him, that someone also kills him. I think that George do not want to have the same problem as Candy with the dog. In my pinion, It was the write thing to do because; anyways they will kill him, no matter what. And if George does not kill him and they try to escape they would find them anyways. George could have a better life without Lenny. In one part is better to George for George, because he had a lot of problems because of Lenny, and because many people do not believed in Lenny.
They do not believed in Lenny because he had problems and act like a kid. Finally that is the way I think, and for me it was the right decision, that George kills Lenny.