Being hit by a car?
I think that affected him big time for the worse. He was not crazy prior to the accident. He has been insane since the crash. Not excusing him but something clearly went wrong after the crash.
Pain drugs?
“... something clearly went wrong after the crash”.
Many of the biography sights on King state that he had issues with alcohol and cocaine prior to the crash. Personal demons, I guess. In fact, many of his bestsellers were written when he was high/drunk.