Why do you think George wasn't mad at Lennie before he shot him?
In part 5 in of Mice and Men, Lennie kills Curley's wife, on accident. Then, in part 6 in of Mice and Men, George finds Lennie hiding in their safe spot by a river. George talks Lennie as if he didn't care that Lennie killed someone and that it was no problem. However, after that George "[steadied the gun] to the back of Lennie's head" (106) and shot him dead. I think George cared about Lennie but knew his "mental" condition was unfixable which is why George was nice to Lennie before he shot him so Lennie didn't grab on to him or attack him. Also, I think that George wanted to live his own life without having to look after and care for Lennie.