Like Roskolnikov states above, this question is definitely open to interpretation. I see George's final action as an act of love and strength. Lenny has escaped his determined fate before with George's help; what George liberates him from is death by the hands of people who don't understand, men who won't try to understand. George understands that this is the end, Lenny is going to be executed, and he is guilty of a crime whether he's mentally aware of it or not. George takes on the responsibility out of love. Lenny's death is quick, instantaneous, and it's performed while Lenny is imagining the farm which is his 'one' happy dream.