In the end, Mr. Wilson was the one to "crack up." There was another mentioning of Doctor T.J. Eckleburg, only this time it was Mr. Wilson who gave his thoughts on it. He saw the empty face on the billboard as God, watching down on them, and seeing Myrtle's infidelity. When Mr. Wilson told Michaelis about this, Michaelis just assumed he was going a little crazy after the loss of his wife, but others also saw him "acting sort of crazy." Which all led him to shoot Gatsby, believing that he was the one to kill Myrtle.
At the end of the book, when Nick ran into Tom, he realized something: they see their actions as justified. Tom thought he was doing the right thing by telling Mr. Wilson about Gatsby, and Mr. Wilson thought he was making things right by shooting him. Nick then felt as if Tom were a child, tying in with the theme of identity, and how what one person sees as right vs. wrong might be totally different from how someone else sees it.
At the end of the book, when Nick ran into Tom, he realized something: they see their actions as justified. Tom thought he was doing the right thing by telling Mr. Wilson about Gatsby, and Mr. Wilson thought he was making things right by shooting him. Nick then felt as if Tom were a child, tying in with the theme of identity, and how what one person sees as right vs. wrong might be totally different from how someone else sees it.