The death of Myrtle was directly caused by Daisy hitting her with Gatsby’s car, but that was just an accident. The true cause of her demise was her own greed and lust for material things. She had everything she needed in life with George. He loved her and took care of her. If she would have only come to her senses and realized this, she would not have run away from him and get hit by Daisy, whom she most likely thought was Tom.
George’s suicide was the result of his own sadness over Myrtle’s death and her want to leave him.