Did George do the right thing by shooting Lennie? Why or why not?

Really we have to say yes. Lennie didn't have many options. There was basically a lynch mob out to get him led by Curley who is basically a sociopath. The best case scenario would have Lennie locked up in a cage somewhere and as Slim said, "That ain't no way to live." George doesn't want to shoot his friend but turning Lennie around and reciting their farm dream (then shooting him) was the most humane thing to do.