These are reasons as to why he would not have been executed or jailed for his crime, well at least not jailed forever. What we don't see is how George lives with it afterword, racked with the guilt of what Lennie has done, and having to put the man down that he believed capable of proving his knowledge of him wrong.
George would not have known what would have happened to Lennie if he was put on trial. The reason I agree with this statement is because if Lenny was put on trial for murdering Curley's wife, then the jury would realize that he is mentally handicapped and he should be put into a special home, or a mental facility.
They both worked at a farm together. In a hard time like that, everyone only focuses on how to earn money, how to survive. Just as it is useless for a mouse to try and protect its home from the blade of a plow, so are the human efforts and dreams for the unattainable in the face of natural and economic calamities.
George must have been thinking that the authorities were either going to kill him, lock him up, or that Curley would seek out revenge.
It was a total accident that he did kill her, because of Lennie's mental state. Lennie may have looked burly and mean, but he was a kind-hearted soul who never meant any harm to Curley's wife.
These are reasons as to why he would not have been executed or jailed for his crime, well at least not jailed forever. The murder in the barn wasn't intentional, nor were the times he killed the animals.
From the beginning, George seems to come off as a little brutal and inhumane in the way he speaks to Lennie at times.