"the day after Caseys and Steve murders Billy said to Stu "Nobody said you were guilty of the murders"."
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Yeah, I noticed that one as well.

What made the reveal smart at the time was that it was hiding in plain sight. Make the killer or killers so obvious, you write them off as a red herring.
ᴳᵒ ᶠᵘᶜᵏ ʸᵒᵘʳˢᵉˡᶠ

Agreed. I think many viewers were probably suspicious of Billy since he seemed a bit creepy (I know I was). But it never even occurred to me that there could be two killers working together. That was a great twist for the time.
So many stories, so little time.

It was an original twist is the main thing. That is why the film worked.

Also when Stu's talking excitedly about the murders in the first major scene where they're all together, Billy interjects with ' Hey, it's called tact you fck-rag' - we think he's protecting Sydney's feelings but now we know better
Billy similarly chides Stu when he enters the house party.

Yes, when watching the movie again and knowing who the killers are, some hints are noticeable, but those things don't really mean much if you're watching the movie the first time around.

You do know in a whodunnit every character acts like they are guilty throughout the film.. Every character had similar moments you can look back on