George understands Lennie's mental limitations and accepts the fact that he cannot be held responsible for his acts. He also has seen the tender side of Lennie - who like a child needs contact with soft things; whose need to "cuddle" is not even sexual but rather stems from a visceral need to love and to be loved.
George knows that Lennie "didn't mean it," but he also knows how Lennie can get out of control when provoked. Curley's wife told him he was "like a big baby" but unfortunately didn't see the dangerous side of his 'infantile' nature until it was too late.
As when they were escaping from the town of Weed, George doesn't blame Lennie, whose limited reason is no match against his brute strength.
George is not angry with Lennie despite the trouble he has created by killing Curley's wife because he understands Lennie and his mental condition, his inability to control his strength and actions. George knows he didn't mean to kill her, he just wanted to touch something soft but got carried away and lost control, which again is not his fault.