George is ambivalent in his feelings about Lennie. He feels obligated to take care of him because of a promise to Aunt Clara, but he resents the burden of responsibility. He also realizes he is taking a risk because of Lennie's tendency to kill small animals and because of the really serious trouble Lennie got into with a girl in Weed.
"Jus' wanted to feel tht girl's dress--jus' wanted to pet it like it was a mouse---Well, how the hell did she know you jus' wanted to feel her dress? She jerks back and you hold on like it was a mouse. She yells and we got to hide in a irrigation ditch all day with guys lookin' for us, and we got to sneak out in the dark and get outta the country."
George senses that more serious trouble is brewing with Lennie. He is starting to take an interest in girls.
There is something ambiguous about Steinbeck's plot. George instructs Lennie to come back to this riverbank campsite and hide if he gets into any more trouble. The purpose would ostensibly be for the two of them to make their getaway as they did in Weed. But George ends up killing Lennie instead. And the fact that he stole Carlson's gun and brought it with him to their hideout shows that he intended to kill Lennie from the time he left the bunkhouse. He doesn't even try to lead Lennie away to safety. He has given up on Lennie. He probably gave up on him from the moment he realized his partner had killed Curley's wife. What is actually happening is not that George is saving Lennie from being tortured and killed by a lynch mob, but that George is peforming an act of euthanasia.
In Chapter one, George indicates that he is getting stressed out from trying to be Lennie's keeper. After Lennie kills Curley's wife in the barn, George must have given up on Lennie and decided that he had to be put away. He couldn't go on protecting a potential killer. He couldn't go on taking Lennie from ranch to ranch and hoping that Lennie woudn't kill again. He doesn't even try to escape the mob with Lennie, as they probably could have done in the dark. Why would it be any harder to escape the mob at night then it was to escape that other mob in Weed in broad daylight?
George was not only killing Lennie out of friendship and compassion, but out of a desire to rid himself of an unbearable burden and a sense of guilt for harboring a menace to society. George must have felt equally guilty with Lennie, or even more so, when he saw the dead body of that young girl in the barn.