Why does George shoot Lennie?

Really George had little choice. Lennie had killed Curley's wife. He didn't mean to but like all things soft, he smothered her and inadvertently broke her neck. George knew the mob was out to lynch Lennie and that at best, Lennie would end up in a cage. George kills Lennie out of compassion to avoid such a terrible fate.

'You had to do it, George,' said Slim. Pg 84