Wilson pulls the trigger

Tom is behind it

"I have a way of finding out"(Fitzgerald166)

"Acting sort of crazy"(Fitzgerald168)

"...the gardener saw Wilson's body a little way off in the grass"(Fitzgerald170)

Thinks the driver of the car is someone Myrtle knew, who she's having the affair with. Confides in Tom because he was there and they are "friends." To get Gatsby out of the picture and eliminate the Gatsby&Daisy possibility he tells Wilson where he lives.