could billy and stu have gotten away with it?

No. There was way too much evidence they didn't think about.

1. If Sydney's dad had been tied up and gagged for two days, police and forensics would be able to tell. They'd see tape marks on his mouth from his gag and probably on his arms and legs even though the tape was over his clothes, so they'd know he'd been tied up for a long time which would cause suspicion.

2. They planned on shooting Sydney's dad and making it look like a suicide, but I guarantee they wouldn't be able to fool an ME once an exam was done on the body. It's hard to face a suicide with a gun. ME's can tell when someone tries to make it look someone shot themselves but didn't. Billy and Stu probably would have just held the gun close to his head and fired, not realizing there would need to be gunpowder on her dad's hands and the gun would have to land in exactly the right spot. Plus, they'd find both of their prints and the fake blood on the gun, proving they both held it. That would poke a huge hole in their story.

3. Billy was covered in fake blood. The police would make him see a doctor, who'd be able to tell most of the "blood" wasn't real. That would raise suspicion.

4. Most importantly, there's no way Billy and Stu would be able to keep their act and stories straight. They did get away with killing Maureen and framing Cotton and did good at not getting caught for killing Casey and Steve, but by the end of the movie both of them were batsh-t crazy. They had completely lost it. If they were the only two survivors of a massacre, they'd be grilled for every detail. They'd either mix up what they were saying, let something slip, or let their true personalities get out. Billy did bet past being interrogated before by the police, but he knew his phone didn't make the calls, Stu would call Sydey which would make people think he couldn't be the killer, and he planned that all out. He had no alibis or outs after this. As soon as the police discovered the first three points and confronted the two about them, they'd screw up.

I really doubt they would have gotten away with it, they were inept and negligent on too many levels to have gotten away with it scot free. But if they did, I reckon they'd have only gotten away with it for a short time, perhaps a few years and their case would have made to an episode of Forensic Files haha.

"could billy and stu have gotten away with it?"

I think they could have, if it weren't for those meddling kids!

It could go either way.

People are citing the strength of forensic evidence. But it probably wasn't as advanced in '96. I'm no expert so I have no clue, but that seems reasonable.

Plus, their story would've been so shocking and terrible as is, it probably wouldn't have crossed anyone's mind that it was a lie, at least for a while. They were already suspecting Neil.

I feel like their plan and story for after could've easily fallen apart, but it would require some chance thinking (for lack of better term) on too many other people, where it would've held up.

If/when it had fallen apart, I feel like it would've been more in the vein of Stu drunkenly mentioning to someone else a few years later something like "Oh yeah, that, well, there's some more to that than people know, but that's all I'm saying," and it would've gone from there, the other person telling the cops that or something.

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No, they would have been screwed due to forensics and how they kind of made up their plan on the fly.

#1) The guy they're trying to pin it on, Sidney's dad, would have bruises from his restraints around his hands, and probably from the duct tape they kept over his mouth. Forensics would look into this, and even ask Stu and Billy about it after them surviving the incident.

#2) Their ending in mind was Sidney's dad kills Sidney, and then shoots himself. The gun, however, came from Dewey, and Dewey wasn't in Stu and Billy's plan. I mean, yes, the dad could have just slit his throat or something, but the fact that they didn't stick to the plan and decided to use the gun instead of the knife was stupid on their parts. Their fingerprints would have been found all over the gun if they didn't wipe their prints off properly. The knife, I can only hope they wiped off properly if the knife was the original method for the dad to kill himself.

#3) Gale's camera was still in the house. It would have shown Billy shooting Randy in the hallway, Stu with the recorder in hand changing his voice. Police would investigate and see that the camera was broadcasting to the TVs in Kenny's van. Now let's pretend Billy and Stu just carried through with the plan without explaining to Sidney whats going on. Would the cameras still be found? Yes, because they both didn't anticipate Gale having a camera hooked up inside the house, which is why Stu had to make sue she was dead when she crashed the van. He didn't though, and she would survive and tell the police to check the footage gathered from camera.

#4) These idiots were stabbing themselves to appear to have been attacked before they could kill Sidney and her dad. Hell, they started stabbing themselves before making sure Randy died from the gunshot wound. Jesus, Dewey was unconscious for god knows how long and managed to live. And again, they thought Gale died when she wrecked the van when she didn't. Let's say they disarm the gun and keep it on them, then proceed to stab themselves, then Sidney and her dad. Gale would've walked in, and neither one of them would have had the strength to chase after her if she were to run away from them.

#5). If somehow by some miracle Stu or Billy managed to pull all of the aforementioned issues above off, the corn syrup on Billy's shirt would be suspect.

There's a possibility and I could go into detail on how and why but I think it defeats the purpose because they made it clear that wasn't their plan. They wanted it to be one big huraah. They were willing to go out with a bang for the plan on that night. It was heavily clear. Also a bit of bad writing as they were so clever and precise before the ending but when it got there they seemed a lot more clumsy and foolish almost to the point that'll make you think it wasn't them the killers.