George had two decision to kill Lennie or let Lennie live and be killed by Curley and the mob or at least be locked in a cage, as slim said' that ain't no way to live' ( page 38). Since Lennie is George best friend, who took care of him since his aunt Clara died, George did not want to watch his friend be tortured and killed in a harsh manner, which explains why George took it into his own hands. Now George could of let Lennie be locked away but he probably knew Curley wouldn't let him get away of him killing his wife and even so Curley hated him from the very start of the book.
George had to shoot Lennie because he had no other way to save Lennie. If Curley had shot Lennie,George would feel like Candy as Candy had said ' I ought to shoot that dog my self, George, i shouldn't ought let no stranger shoot my dog ( page 58) when Carlson killed Candys dog.One might believed that George should not have shot Lennie because it was wrong, however George killed Lennie because it was the right thing to do.George shot Lennie for Several things, he shot Lennie to protect him OR to save him because Lennie didnt know his own strenght . Lennie was killed by George out of love and respect for him as a normal person, not because he always got them in trouble and always mess things up for them.
George also had to kill Lennie becuase as Lennie said ' I got you to look after me and you got me to look after you' ( page 13). That is anther reason why George felt that he had to shoot Lennie, because he had to look out for him, He had to do what was best for Lennie and shooting him was