I am just speculating here but I think that he was most likely targeted for a robbery. He was probably followed for some time, I don't believe that someone saw him driving in his nice car and just decided to rob him right then and there. I'm not saying it was those two guys, because it could have been anyone. The robbery might have even been planned for a while by someone who knew he didn't have security and found where he lived and what kind of car he drives. And since he was most likely shot in the back of his neck or head, I think that maybe they pulled up behind him and just shot him through the back window while they were still in the car, one got out and grabbed the bag, got back in the car and they left. This could have all happened in less than a minute. They probably didn't even know or think they killed him.