In Chapter 8 of The Great Gatsby, what does George Wilson think happened to Myrtle?
George believes that Myrtle was murdered.
Though George Wilson does not seem to know immediately who was driving the car that killed Myrtle, he says to Michaelis:
"He murdered her."
Wilson goes on to say:
"It was the man in that car. She ran out to speak to him and he wouldn't stop."
When the sun rises that morning, George Wilson leaves on foot to track down the driver of the car. As we learn later when Nick confronts Tom about the events of that day, Wilson comes to Tom's house and Tom tells Wilson that it was Gatsby that ran down Myrtle.
Tom knows, of course, that it was his wife, Daisy, who was driving Gatsby's car that night. It was Daisy that killed Myrtle (her husband's lover).
By directing Wilson to Gatsby, Tom is doing a few things. Partly, we can presume he is protecting himself from being discovered in his affair with Myrtle Wilson. More to the point, he is deflecting blame from Daisy and thereby saving her and himself from punishment and trouble. He is also essentially creating a scenario wherein Gatsby will be killed.
Tom suggests in the end that Daisy did not tell him that she was driving the car that killed Myrtle, but Tom does admit that he knew what would happen to Gatsby once Wilson was given his name.
"The fellow had it coming to him."
Without pity or remorse, let alone guilt, Tom admits to using Wilson's belief that it was Gatsby who killed Myrtle as a way of killing Gatsby. Tom claims it was self-defense and also suggests that he was just telling Wilson the truth.
The incredible indifference shown by Tom in this passage links him to the moral emptiness that so often characterizes his ilk in the novel. Money insulates him from punishment and so insulates him from any sense of wrong-doing. Wilson's character, deranged as he is, still clings to a sense of moral order.
Wilson takes on the wrath of divine retribution as he stalks Gatsby, if we are to read his understanding of the billboard of T.J. Eckleburg as any indication of his state of mind.
Wilson: “God knows what you’ve been doing, everything you’ve been doing. You may fool me, but you can’t fool God!”
Standing behind him, Michaelis saw with a shock that he was looking at the eyes of Doctor T. J. Eckleburg, which had just emerged, pale and enormous, from the dissolving night.
George, in trying to make sense of the events that preceded Myrtle's death and in shock because of the terrible event, comes to the conclusion that Myrtle had been having an affair with Jay Gatsby. He sees what he considers to be evidence proving this in the memory of Myrtle's injuries after traveling to New York City with Gatsby and the Buchanans and Nick a few months previously. He sees further proof in very expensive dog leash that had been hidden in Myrtle's desk drawer, assuming that Gatsby bought it for her, looking forward to the time when they could have a shared pet. George reaches the conclusion that Gatsby purposefully hit and killed Myrtle while Gatsby was driving the yellow car, and he resolves to get revenge.