There are many different justifications one could come up with regarding George killing Lennie at the end of Steinbeck's novel Of Mice and Men.
1. George knows that if Curley and the other men find Lennie that they will kill him. George also knows that Curley is still angry with Lennie for breaking his hand and that Curley will not be killing Lennie as a "sympathy" killing. Instead, Curley will, instead, kill Lennie as an act of revenge.
2. George knows that it would only be right for him to be the one to take Lennie's life. One could justify that George has been taking care of Lennie for a very long time and he wishes to be the one who took care of him up until the end.
3. One last reason why George could be considered "right" in killing Lennie is that George has finally come to his wit's end with caring for Lennie. Knowing that Lennie will surely mess up again, George is worried about having to get Lennie out of trouble for the rest of their lives. George is simply tired of taking care of him.
A reason can include the fact that if Geroge didnt killl elnnie then Lennie could have been treated very bad if his matter was to go infornt of the law just because in that time period people with mental illnesses were not treated as people but rather as monsters so Geroge was just trying to help a 'friend' that had done alot for him and he just never wanted to see him suffer more.