In this scene, Lennie is choking Curley's wife because she was being a flirt. He does not kill her. She had already been bitten by a snake that was around the farm. The snake is crawling around waiting for his poison to kill her.
In this scene, Lennie was so upset that he thinks he killed Curley's wife. Lennie did choke her but he only made the process of her death faster. The snake had already poisoned her and she was going to die anyways and Lennie just made her die faster than she should have because she was poisoned.
In this scene, Lennie was so upset that he is scared of George and him being mad at him so he runs and never comes back. He takes a tent with him, wood to make some fire. There he meets lots of animal friends and he lives happily ever after. George thinks he died.