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

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.