Tony was a sociopath, or at least close, his thin justifications for Chris murder, remember him going on and on about the by seat, were just his defense mechanisms to rationalize his behavior. The way Tony always did. He killed Chris because their relationship was failing, though not altogether broken, the resentment and bitterness between the both of them was their for years, worse after the Ade incident. And Tony was also really affected by Chris portrayal of him in that 'Cleaver' movie. He thought Chris despised him, and would never forgive him for a number of things. And he was probably right, but if Tony wasn't Tony, they could have worked through, Chris still loved him, and looked up to him, crazy. All Tony had to do was be there for Chris, but the selfish, self deluded fukk couldn't do it. Look at Tony's treatment of Chris at his barbecue. And then christening.
Also, if you watch closely, Tony kind of realized Chris is coked up, in those minutes right before the crash. Chris is distracted, fidgety, talking weird, but Tony, is talking about 'Smell the roses' and trying to see the positive, but when Chris confesses, Tony just methodically kills him, and solves a number of his problems. It wasn't really business why Tony killed him, it was pettiness, and resentment, anger, jealousy, disappointment, disillusionment. Sad way to go.