While it can never be considered right to kill anyone, sometimes it’s necessary, such as in a war situation, or if someone attacks you or your family and there’s no realistic alternative to using lethal force. On balance, one would have to say that George was placed in a similar situation with regards to Lennie. George vowed to Lennie’s Aunt Clara that he’d take care of him, and by killing Lennie, George thinks that’s exactly what he’s doing. Although Lennie didn’t actually mean to kill Curley’s wife, and despite the fact that he has the mental capacity of a child, it’s almost certain that he’ll be convicted of murder. And at that time and in that place, that would have meant the death penalty for Lennie. So although we cannot in any way condone George’s actions, it could reasonably be argued that he didn’t have much choice in the matter.