The reason why you can't figure it out is because it wasn't him who committed the murders.
No fingerprints/blood/sweat/fibres/saliva . No forensic evidence whatsoever or any other evidence.
You say.
He was certainly cooking up something with that 3 a.m. [3.15 a.m./3.30 a.m.*] phone call to Julie And Friends. Douglas answered. Then he didn't. It turns out it was Julie who answered after all. Jeremy, sorry Jerry, had told her 'tonight's the night' [did he like Rod Stewart?]. Helpfully, Julie was up later for that early hours of the morning phone call. Good of her.
Then Julie tells Susan that Jeremy did it, but Susan doesn't mind if Jeremy and Brett come along to her 21st. birthday party. Susan must be a very broad-minded gal. Personally, I would have been running to the nearest police station, and if I caught sight of Jeremy, I might have made a citizen's arrest.
But Susan's a modern, independent woman and mass murder is water off a duck's back.
Then we have our friend, Steve, who posts on here defending Julie as if his life depends on it, and he's touchy about Colin too. Touchy touchy. Mustn't upset Julie or Colin!
It's all very curious.
Anyway! Must dash...