Hearing Willie tell us Alissa Hyatts's sister was murdered that morning by her boyfriend, then indicate he still believes she went on that trip anyway, to have Shanquella beaten to the point they broke her neck and spinal cord, because she and everybody else were jealous of her success, after she paid for everybody, sounds like something he don't even believe himself at this point.
If what he said so far is true, and knowing there's more to this story, then I can think of two other possible motives right off bat. But I don't think people can handle it yet.