What is the message; of mice and men?
Sometimes the right thing to do is not the easy thing to do. Lennie kills George because it is kinder than to let the mob do it. It’s better that someone who loves Lennie and understands him kills him. George did not kill out of hate, but out of the understanding that Lennie could not live in the world without destroying the world around him. The men are menial laborers in an uncaring world. The most caring thing that George could do was to kill Lennie rather than let his innocent (but destructive) soul be killed by the angry, uncaring mob that would come after him.