I'm normally not one for conspiracy theories, but it does seem a little coincidental that Diana's bodyguard, with whom it was rumoured she was having an affair, was removed from his position and was killed three weeks later in a motorcycle accident.
I'm not convinced that Phillip would have made the decisions, though; Diana's actions were reflecting poorly on the rest of the Royal Family and I think it more likely that someone else (maybe someone who was responsible for maintaining their public image) made a private decision to have her killed.