D.O.A ending / spoiler
Hal Petersham was the one who poisoned Dexter Cornell. He was jealous of how naturally talented Dexter was in making novels. And when he discovered high schooler Nick Lang had the same 'gift' in creative writting, he decided to steal Nick's story; 'out of whack' and pass it off as he's own. That meant killing off everyone who knew diffrent...hence the murders. When Hal tries to drop a typewritter on Dex's head he is shot by Dex with Cookie's gun (she was accidently shot dead by brenard who was then chucked into the tar pitts by Dex...thus burring him alive) and crashes out of a window to his demise on the school yard below (landing on the exact spot where Nick fell onto.)
