George is legally guilty of murder for killing Lennie. Steinbeck evidently wanted to make it plain that he would not be accused or put on trial, so he used a few lines of dialogue between George and Carlson at the very end to establish that George supposedly shot Lennie in self-defense and this would be reported to the authorities. Carlson assumes this was actually the case. He asks:
"Did he have my gun?"
George takes the opportunity to clear himself:
"Yeah. He had your gun."
And Carlson hands him the alibi:
"An' you got it away from him and you took it an' you killed him?"
Actually George got Carlson's gun himself when he was leaving the bunkhouse to look for Lennie, but the reader doesn't know this for sure until George takes it out of his side pocket at their meeting place by the river. Since he knew exactly where Lennie would be hiding, he got there well ahead of the others. He may have decided to kill Lennie with Carlson's gun even before he reached the spot where Lennie was hiding. He waits until the last minute because it is so hard to kill his friend, but the approaching mob finally forces him to pull the trigger. Steinbeck has already taken pains to establish that Curley, who is leading the mob, has a violent, sadistic nature.
George may not even have to lie to the police. The men may tell them that Lennie must have stolen Carlson's Luger from the bunkhouse, and Carlson will tell them that George took it away and killed Lennie in self-defense.