Why didn't Lennie just let Curley's wife go?

In chapter 5 I was very interested when Lennie knew he was hurting Curley's wife but wouldn't let her go. "I don't want you to yell. You gonna get me in trouble jus' like George says you will. Now don't you do that." And she continued to struggle, and her eyes were wild with terror. He shook her then, and he was angry with her. "Don't you go yellin'," he said, and he shook her; and her body flopped like a fish. And then she was still, for Lennie had broken her neck (91-92). Lennie didn't let go of her because he was scared and frightened and didn't want to get in trouble.