It’s true that George left Lennie unsupervised and he also killed his best friend; however, George was justified in killing Lennie.
In fact, if George had have let Lennie live, Lennie would have suffered. If Curley killed Lennie he would’ve been tortured. Curley wanted to shoot Lennie in the guts so he would have suffered and bled and been in a lot of pain. On page 96 there is proof, Curley states “I’ll shoot ‘im in the guts”. If caught by someone other than Curley or the people on the ranch he would have been locked up in a mental institution. He would be treated really poorly because the asylums treat everyone horribly. They just throw you in a filthy room with everyone else that has a disability and let them roam free. On page 97 Slim says to George, “…lock him up an’ strap him down and put him in a cage.” And lastly if he was imprisoned, he would be tortured because of his disability: mental retardation. On page 97 it shows George saying, “Couldn’ we maybe bring him in an’ they’ll lock him up?” his disability would make him slow and people would pick on Lennie until he broke down.
Furthermore, Lennie has killed many things before. What was to stop him from killing anyone or anything else? He used to kill mice back when he was little because they would bite his fingers because he loved to pet soft things, and he would attempt to pet them. So he would pinch their heads in return. On page 10 it says “I’d pet ‘em and pretty soon they bit my fingers and I pinched their heads a little and then they was dead because they was so little.” He also killed a young puppy. On page 87 it says “I was just playin’ with him…an’ he made like he’s gonna bite me…an’ I made like I was gonna smack him…an’ I done it. An’ then he was dead. Lennie also surprisingly killed a human being too! He killed Curley’s wife with no instrument at all. He just used his own two hands and broke her neck, just by shaking her. On page 91 it says “…for Lennie had broken her neck.”