Both deaths are ironic.
Daisy unwittingly kills her husband's mistress. She didn't even realize whom she had killed.
Wilson intends to kill the man who was having an affair with his wife (which is actually Tom), but Tom sends him to Gatsby's house instead.
In The Great Gatsby, Myrtle and Gatsby are both Alazons, impostors who move from rags to riches and who live above their socio-economic classes.
Myrtle jumps classes through infidelity. Myrtles lives in the Valley of Ashes with George where she is low class. But, with Tom in their apartment in New York, she lives like the upper-class Daisy. She buys expensive dog collars and throws posh parties.
Gatsby jumps classes through criminal activity: bootlegging and gambling. He comes from a modest background, but after he meets Dan Cody, he reinvents himself and changes his name. He partners with other gangsters to make money during Prohibition.
Also, both are killed by mistake. Myrtle is run over by Daisy, but Myrtle thinks it is Tom's car come to rescue her. Gatsby is shot in his pool because George think he is Myrtle's lover and the one who ran her over.
In the end, the American dream has no room for opportunists and impostors. Those who go from rags to riches end up in the grave.