Lennie does not understand what he is fully capable of. He is never able to reach
his full potential at anything because he underestimates his power. Lennie is always killing things. This is seen when Lennie kills Slims puppy. Puppies are not too fragile, it takes a lot of power and strength to hurt it, let alone kill it. Lennies kills the puppy by hitting it. He claims that the puppy seemed like he was going to bite him. He took “disciplinary” actions on the puppy but since he doesn't quite understand his strength, he ends up killing the puppy. Lennie not being able to recognize his strength is also illustrated when he breaks curleys hand. The men have to make Lennie let go because hes not understanding what he is really doing. Some may think that lack of understanding yourself is no reason to be killed but Lennie didn't just not understand himself he doesn't understand the world around him and how it functions. You cant live at ease or really live at all without fully understanding what you are capable of doing and what you are surrounded by.
The world we live in would never understand the simplicity of lennies mind, we would never look and try to figure out why he commits certain crimes. George is trying to save Lennie from a world that will never understand him. For instance, Lennie kills Curley’s Wife. He kills her because she offers him a chance to touch her soft hair (Lennie is attracted to soft things). Once lennie started to stroke her hair he stroked harder ultimately hurting her. This causes Curley's Wife to scream. Lennie is really startled. The simple mind that lennie has tells him not to let go, in fact it basically tells him to hold on to the hair tighter and get Curley's wife to quiet down. In the midst of all of this climatically Lennie breaks curley's wife's hair and kills her. He did not do it intentionally but as said before his mind did not analyze the situation and give him the correct thing to do. Another example is in Weed, where Lennie is accused of rape. This happens when lennie spots a soft looking skirt and decides to touch it. When the woman begins to panic and yell Lennie’s thought process goes blank and decides to hold on. the startled woman doesn't know lennie nor will she understand his “condition”. Consequently she accuses him of rape. Forcing Lennie and George to escape town and go find work before getting caught. George did not want to see Lennie suffer in a world that would always see him as a bad guy and never as the man he really is.
Georges intentions on killing Lennie are good. Hes killing Lennie because, Lennie does not understand himself or what he is capable of. Also because he wants to save him from a world who will always view him as a bad guy. Some believe that killing no matter what the reason is wrong, evidence shows that killing is better than making them slowly suffer. Slow painful and scaring or fast unexpected and painless?