I believe that George was unavoidable in killing Lennie because Lennie was doomed to death. As we can see throughout the story, George is a good friend of Lennie and he cares about Lennie, not like other guys. For example, George always told Lennie about their dream of living in a place with freedom.
Also, judging by George saying “Lennie, for God’ sakes, don’t drink so much. You're gonna be sick.”, George cares about Lennie. It was certain that they were precious to each other. If so, was killing Lennie truly the best choice for George to make? George shoots Lennie to save him from Curley and to liberate him from the world, not because George was mad. If George had not done that, Curley and the others would have found Lennie and inhumanely executed him. If George lied to the other guys and helped Lennie escape, maybe in the future he might get into worse trouble because, as the book delivers to the readers, it is difficult for the socially underprivileged, such as Lennie to survive in our society.
These circumstances reminded me of Candy when Carlson killed Candy’s dog, and he says, “I oughtta of shot that dog myself, George. I shouldn’t oughtta of let no stranger shoot my dog.” When George made Lennie turn around and told him to look across the river, George started to talk about the thing Lennie loved the most, tending the rabbits and going to the place where they dreamed about all the time. George desired Lennie to go to heaven in peace without knowing what would happen. At last, he says, “No, Lennie. I ain’t mad. I never been mad, and I ain’t now. That’s a thing I want you to know.”