Possibility 5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—who is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level.
If we're the most advanced civilization right now, that will be us in a few million years. We're kind of dicks.
Exciting for a movie plot I guess. But, what could they win from exterminating another inferior intelligence?
By your context, they are already inferior, and thus, no threat to the apex civilization.
It's been suggested by Robin Hanson (who coined the name "Great Filter") that an ethical super-civ would see a galactic war with another super-civ as the worst thing possible, and so wipe out other civs to stop that worst thing possible,
I don't buy it, FWIW, because "ethical" doesn't overlap with "wipe out all other life" in my book. (Although you could certainly, say, have some system that stops any civ from leaving their planet by dropping rocks on them each time they launch something into orbit.)
But you can search Hanson's blog for more talk about this if you want to see more discussion.
If we're the most advanced civilization right now, that will be us in a few million years. We're kind of dicks.