How best to Showcase the Treachery of Evil (tm)?
My suggestions all come from game experience.
The kill-the-Fake-Big-Bad-cutscene was around the middle of The Watch House S2. I also had one group of villains wipe out an unconnected group of villains in S6, also off-camera. It helped to establish the new Big Bads' scary credentials, and in both cases killed off human villains who the PCs wouldn't be willing to kill.
The play-the-villains one-shot was the first (or second) convention game I ever played, one I still recall fondly. We were archetypal supervillains meeting to discuss a plan to Take Over The World, but we each had enemies at the table and other secret agendas, one of us had sold his soul to a demon who naturally turned up at an inconvenient moment, another was secretly Brooding Black-Cloaked Hero in disguise, and so on. It duly ended in violence, recriminations and blame-passing in-character. The tricky part was not having that happen right away.
I ran something somewhat similar at Propulsion the year it was KillCon, where there was a suggested PVP theme for all the games. Star Wars with six would-be Sith apprentices in their final trial. The trick there was that the trial was dangerous enough that no one apprentice could make it all the way through alone, so they had to work together... mostly...