George and Lenny had to move from place to place not only to find work, but because Lenny was often getting himself into situations, predicaments or trouble. Not that Lenny was inherently bad or evil, but that he was a big man, strong as an ox as voiced by the foreman that Lenny could out-work several men. Lenny was lacking in discretion because he essentially had the mentality of a six year old and this mentally challenged condition did not allow him the the wherewithal that he might be over-exertive at times. In this final instance, Lenny wound up accidentally killing a puppy and Curly's wife in an attempt to quit her. The same Curly that belittled and provoked Lenny into defending himself, at George's behest, that culminated in Lenny crushing Curly's hand. George knew he couldn't bail Lenny out of this one and they weren't going to be able out-run Curly, his posse and their hounds. George also knew Curly had a bloodlust for vengence & this would be his justification to brutally kill Lenny in the most torturous fashion. George also remembered Slim's suggestion of Lenny being instutionalized and Candy's regret of not shooting his old dog himself. As George caught up to Lenny at their secret, pre-arranged meeting place, you could feel the emotion in the passage when George tells Lenny to think about the farm they came so close to finally aquiring. Leading Lenny to believe they soon would and that Lenny would be charged with taking care of the rabbits. What a last vision to instill in Lenny's simple mind who was non-the-wiser of the gravity of the situation. Then, as Lenny was kneeling and re-counting how he would hold, love and pet the rabbits, George knew he had had to pull the trigger aiming, at the base of Lenny's skull, to give him a better fate than he would have met at the hands of Curly.