I was examining the crime scene. This dude was explaining how he and his friend were being terrorized at night by a masked killer.
It turned out there was some kid bullying them named Colin. The masked killer was actually Colin’s dad, and had been showing up to their house every night, trying to pick the lock apparently so Colin could easily get in and kill them. The noise from his lock-picking had been causing the protagonist’s friend to get these weird sleep tics which could be heard from the other room so that was a big red herring for a while?
Eventually the dad returns, has a scuffle and kills the friend. Presumably the result of this was the crime scene I was investigating. Later, Colin shows up to finish the job. He has a fight with the protagonist. Both suffer injuries and end up on the ground, nearly dead. The protagonist makes to kill Colin, and is so confident he’s going to do it that he starts making a cool one liner before the finishing blow. Then mid-sentence Colin cleanly slices his head off with a hack saw and it really surprised me!