In Steinbeck's Of Mice and Men, why did George kill Lennie?
George promised Lennie's Aunt Clara that he would take care of Lennie after she died. Lennie's mind is like that of a child's so he couldn't take care of himself if it weren't for someone looking after him. When Lennie accidentally kills Curley's wife, Curley vows to kill him. George knows that if Curley gets to Lennie first, he won't just kill him, he may do something worse before he allows him to die. George feels it's his duty to deal with Lennie in a more humane way.
There's also a parallel connection about George's responsibility with Lennie and Candy's old dog. Carlson actually goes out and kills Candy's blind, old dog for him. Afterwards, Candy says to George that he should have taken care of his dog instead of letting someone else do it for him. George must feel the same way about Lennie. When Candy shows George the woman's dead body, they both know Lennie had done it. George says the following:
"Lennie never done it in meanness. . . All the time he done bad things, but he never done one of 'em mean. . . We gotta tell the guys. They got to bring him in, I guess. They ain't no way out. Maybe they won't hurt 'im. . . I ain't gonna let 'em hurt Lennie" (95).
Lennie was like Candy's dog: both were killed to prevent them from further suffering. After killing Curley's wife, Lennie wasn't going to live to be brought up on charges because Curley grabbed a gun and set out to kill him. It was difficult to do, but George killed Lennie because he felt it was his duty to save Lennie the horror of being faced with a mob of men who would make his death worse than it needed to be.