Why didn’t Sidney trust Randy?

I’m rewatching the whole series and I just realized something: at the point towards the end of the first movie, right before they reveal the killer there’s the moment where Randy and Stu are both accusing each other of being the killer to Sidney, and Sidney doesn’t know who to trust so she just locks them both out. But this doesn’t make a whole lot of sense, because earlier she saw the footage of Ghostface sneaking around behind the completely oblivious Randy, so why wouldn’t Sidney trust him then? She has no reason to think there are two killers at this point and even if she did, Randy clearly didn’t even know Ghostface was there as Ghostface was sneaking around him, so why didn’t Sidney trust him?