Whether it was the drugs, the testosterone, or a combination of the two we will never know, but at one point on their manic race through the streets of London Tara completely failed to notice a red light ahead and ploughed into the back of the turning vehicle. Tara was killed instantly although Suki survived. Tara apparently swung the car towards him to save her.
The details of this incident have been widely reported, both at the time and since, but what has never come out is that McCartney was present at the crash. Paul, despite his drugged state, quickly realised that he could not be found at the scene and removed himself with great haste. The event left huge psychological scars on McCartney and as part of his penance he included the funeral scene on the Pepper cover as his personal tribute and remembrance to his dead friend.
A couple of AC Vermouth-ism's.. (and I may be paraphrasing a bit)
Paul now wears his scars with pride. (Tara) and:
Tara's death was the icing on the cake. (quitting)
"It is quite possible that societies - much like individuals - collectively repress information, concepts, and ideas which would produce high anxiety levels if dealt with consciously."