While George Milton intentionally kills Lennie Small, he probably would not be charged with murder because the other men (with the exception of Slim) believe that George has shot Lennie in self-defense. It's also true that they meant to kill Lennie themselves and so are unlikely to report him. Finally, the ranch is remote and vigilante justice appears common in the area (as evidenced by George and Lennie's flight from Weed). George isn't likely to be prosecuted.
As George and Candy look down at Curley's wife in the barn stall, George reasons that the other men need to be told that she's dead. He knows that Lennie is most likely responsible and says that he will return to the bunkhouse—but he tells Candy to wait until he reaches this building and goes inside.
"Then...you come along and make like I never seen her.... So the guys won't think that I was in on it."
Candy agrees to do this. He waits for a while, then he goes up to the bunkhouse and informs the men. When the other men come into the barn, they race around the last stall; George has joined them, but he has put on his blue denim coat and has a hat pulled down over his face because he wants to avoid scrutiny. The men's attention is on Curley's wife.
As Curley looks at his wife, he calls out, "I know who done it." Slim quietly suggests to George that this scene may be "like that time in Weed...."
Slim sighed. "Well, I guess we gotta get him. Where you think he might of went?"
George stumbles some on his words, but he finally says that Lennie would have gone south because they arrived from the north. Then he urges the excited men, "Don't shoot 'im. He di'n't know what he was doin'." But Curley shouts, "He got Carlson's Luger. 'Course we'll shoot 'im." Curley desires revenge against the man who has broken his hand and now, he supposes, has killed his wife. Because he does not trust George, Curley orders him to stay behind.
However, George sneaks away. In fact, the scene is almost a repeat of his and Lennie's first night as they camped before the fire. However, this time George does not scold Lennie, nor does he object to reciting the dream. As Lennie urges George to buy their land now, George agrees, then he raises Carlson's gun, steadies it, and pulls the trigger. Lennie falls forward; George looks at the gun and hurls it away from him.
When the others arrive, George again studies his right hand, which has held the gun. Curley sees what has happened. "Right in the back of the head," he observes. "Never you mind," says Slim, who has "God-like eyes" (Ch.4) and whose "ear heard more than was said" (Ch. 2). Slim "looks through George and beyond him." He simply says, "A guy got to sometimes." Clearly, his words indicate a deeper understanding than that of the others. Slim reasons that George has shot Lennie in order to prevent his being killed painfully by Curley. Slim also realizes that even if he were put in prison, Lennie would not fare well without having George around.
With this one exception, the other men believe, as Carlson does, that Lennie had the Luger and George got it away from him and shot him.
"Yeah. Tha's how." George's voice is almost a whisper as he affirms this conjecture of Carlson's, and he still looks steadily at his right hand that has had held the gun.
With his deeper understanding, Slim reaffirms his previous remark, "You hadda, George. I swear you hadda. Come on with me." In a gesture of protection, Slim "led George into the entrance of the trail and up toward the highway."
Slim's affirmation that George has been right in what he has done indicates that Slim would also defend George's actions as acting in self-defense, just as the others believe.
George pulled the trigger with intention to kill. He considered what he had to do (retrieve a gun, find Lennie in the exact location he knew Lennie would be in, and position Lennie to be ready to be shot). He had a good intention, but it was an intention to kill. We know this because he shot Lennie in the back of the head, told him the story of having a little place and living off the fat of the land, and he made Lennie as comfortable as he could for the moment of death. This is kind gesture, but it demonstrated to us as readers careful planning. That makes it murder.
If the charge was manslaughter, it would be on accident. There was no accident here, it was purposeful. Sometimes murder can be in the second or third degree. These are in cases of defending one's self or someone else, or being not as sane as one normally is.
I think he knew exactly what he was doing, this makes it first degree.