Chapter 8 of F. Scott Fitzgerald's The Great Gatsby concludes with Gatsby's disappearing "among the yellowing trees." The "ashen, fantastic figure" of Wilson glides towards him. After the chauffeur hears the shots, he, the butler, gardener, and Nick hurry to the pool only to find Gatsby dead on his mattress. Then, Nick comments,
It was after we started with Gatsby toward the house that the gardener saw Wilson's body a little way off in the grass, and the holocaust was complete.
Here, Nick likens Wilson to Gatsby as sacrificial victims to the "holocaust" of waste and materialism and corruption. Devastated by his wife's infidelity, Wilson, who once told his wife that she could not fool God; that "God sees everything," means for these remarks to reflect upon her actions, but they also are relevant to his act of suicide. For, he feels tremendous guilt for calling Mrytle out about her affair as his angry chastisement has led to Mrytle's running into the street where she has been killed.
When Mrytle is killed, Wilson cries, "Oh, God! Oh God!" This remark is notable, also, as Wilson's cries remind the reader of the absence of religion in the upper classes that "smash[es] up things and creatures." George Wilson is a "true product of the wasteland between the suburban world of wealth and New York City." (enotes)
I wonder if you are asking about Wilson's actions at the end of Chapter 8. After all, he dies at the end of Chapter 8 and therefore does not appear in Chapter 9. He is mentioned a little in Chapter 9, but I would think this question makes more sense if it is about Chapter 8.
At the end of Chapter 8, Wilson kills Gatsby and then kills himself. He does these things because Myrtle is dead and because, I think, he found out that she had been cheating on him.
Wilson kills Gatsby because he thinks Gatsby was the one driving the car and he thinks Gatsby purposely ran Myrtle over and killed her. He kills himself, in my opinion, because he does not think life is worth living now that he has found out his wife was cheating on him. He had a pretty sad life to begin with, and now to find out his wife was betraying him probably sent him over the edge.