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Yeah I noticed that as well. Lots of forensic evidence that would not support their version of events. It doesn't really bother me much though, still love those final scenes.

They are two survivors who, together with a lot of other evidence, point to Sidney's father being the killer. What are the chances of the police in a small town dusting everything for finger prints when there's not even going to be a trial because the suspected killer is dead?

The voice changer wasn't even the biggest mistake they made, that was the decision to stab each other before everybody else was dead. There was no way they were going to get away with it after that.

They were sloppy. But that was kind of the point of it. On top of all the things listed above, but they had 3 more people who they assumed were dead, but weren't (Randy, Dewey, and Gail). Randy for sure could have verified that Billy shot him, and that he was running from Stu who he thought was the killer.

Gail also still had her camera rolling in the living room.

And most of all, when he look at Mr. Prescott, they would see tape marks on his lips, and bruises on his wrist, indicating that he was bound. There would also be duct tape marks on his clothes.

But they were probably banking on a small town police force being so overwhelmed by this crime scene that they do very little investigation, or a sloppy one at that. This was the same police they easily fooled a year earlier with one murder. Now imagine them trying to investigate 7 murders.

Movies like this don't really concern themselves too much with forensic-related stuff. It's not CSI. Besides, maybe they thought their cover story would be strong enough for it not to matter, the police would think the killer was dead and there wouldn't be any need for further investigation.