George killed lennie because he knew that if he didn't then he would be locked up in jail/ an asylum or shot by curley. George killing lennie, though it seems otherwise, was an act of love. He wanted Lennie to die peacefully as they discussed their dream, rather than suffering. you could say that it was a selfish act, he could have run away with lennie and only killed him so that he could get on with his life in peace. It's quite a controversial topic, but generally speaking most people say it's an act of love