RIGHT OR WRONG?

I think George does the right thing.

Don't you think George could have done the wrong thing? He had no right to determines someone else's fate like that. Why not just let Lennie go off into hiding somewhere, instead of shooting him. George will regret killing Lennie for the rest of his life.

No, I think he will miss Lennie terribly, and the implication in the book is that George will end up like all the other ranch guys--lonely, directionless, spending his money on booze and women.

As well if he would have let him go, Lennie would have starved or even drank some dirty water from a creek, since he doesn't test the water.