In Of Mice and Men, George is not legally justified in killing Lennie because his act would be perceived by officials of the law as murder.
Though George feels that he executes a mercy killing, much as Dr. Kevorkian felt that he was ending the pain of those in whose suicides he assisted, such a choice to end another's life is not really available to another man. Nevertheless, George feels he must get to Lennie before the others such as Curley, who seeks a terrible revenge for his wife's death as well as for his damaged hand. Carlson also becomes excited by the thought of inflicting pain upon Lennie, whom he suspects of having stolen his Luger.
Later, after the other men catch up with George, the sadistic Carlson wants to relive what has happened to George:
"Did he have my gun?"
"Yeah. He had your gun."
"An' you got it away from him and you took it an' you killed him?"
"Yeah. That's how." George's voice was almost a whisper.
Despite the immorality of George's act within the narrative of Steinbeck, who himself makes no moral judgment, the reader is prone to accept George's shooting of Lennie. It seems more an act of friendship and love than it is murder--the lesser of three evils--as the reader realizes that George cannot bear to think of poor, animal-like Lennie locked into a cell or put into an asylum, or, worse yet, possibly torn apart and shot by the vengeful Curley and sadistic Carlson.
In an act of friendship, wisdom, and understanding, Slim, who sees everything with his "God-like eyes," consoles George for preventing the others from getting to Lennie by shooting him, "You hadda George. I swear you hadda. Come on with me." Slim knows that it is not George's fault that his and Lennie's "well-laid plans" have "gone awry."