Martin Bashir created fake evidence which made Diana believe that the closest people to her were being paid to spy on her.
Imagine thinking that you had established that small circle of trust, and then somebody shows you evidence to prove they are all lying to you every day.
She got rid of her private secretary and her protection officers because of this, and she went to her death not knowing who she had left in the world that was not being paid to report back on everything she did in her private life.
Once Martin created this world of not knowing where it was safe to turn, Diana was always going to be at risk.
She had to look over her shoulder every minute of every day, and he destroyed all of the protection that was in place around her.
It was this that led to the car accident, and if it hadn’t been that car on that night, she was always going to be physically at risk of something happening.
Whilst Martin did not physically cause the crash, he caused Diana to believe that she was in a position where she had no choice but to dash across Paris in a speeding car without her bodyguards present and without the advice of her protection officers, her private secretary, or even some of her loyal friends.
She would never have been in that situation if it was not for Martin Bashir.
This is why William was so angry with Martin, and this is why he requested that the interview never be shown again.
He knows that his mother was speaking in that interview under the premise of something which we all now know not to be true, and that this not only impacted her decision to do the interview, but also impacted all her decision making from there on in.