What does George plan to do to Lennie ? How do you know this is difficult for him ?

George plans to kill Lennie mercifully before the other men have the chance to shoot and hang him.

George took off his hat. He said shakily, "Take off your hat, Lennie. The air feels fine."

George raised the gun and his hand shook, and he dropped his hand to the ground again.

And George raised the gun and steadied it, and he brought the muzzle of it close to the back of Lennie’s head. The hand shook violently, but his face set and his hand steadied. He pulled the trigger.