Myrtle Wilson's death acts as a trigger, changing the direction of the novel. The hit and run creates tension and blame between characters. Daisy was the person who hit Myrtle, but she was driving Gatsby's car. He took responsibility and claimed to be driving the car in order to protect her. Tom, despite knowing the truth, leads Myrtle's angry husband, George, straight to Gatsby to carry out revenge. Because Myrtle died, Gatsby died too.
Even if Myrtle Wilson didn't die, another tragic event would have happened in its place. There was already tension between the characters, starting in New York City when Tom Buchanan and Jay Gatsby fight over who Daisy loves more. The characters were already having issues between them and would have most likely split up regardless.
[P1] Myrtle Wilson's death triggers other events in the novel. [P2] Gatsby took the blame for killing Myrtle Wilson. [P3] Gatsby died because Myrtle Wilson died. [P4] Therefore, the theme of The Great Gatsby is death.