I must say, I fully agree with you. Both Aedale and Cora have no reason to "destroy" Brian (or at least, it isn't personal, like with Treghak or Aridon). As for Kenos... well, he's an Imperial General, so, who knows? However, I'd be surprised if both Brian and Treghak aren't already aware of the machinations of The Circle. But then again, Treghak has more important stuff to focus on, and from what I understood from the epilogue, Brian seems to have retired form plotting (somewhat, since he is still writing to Gardon about dealing with the Dragon Queen).
What intrigues me the most about The Circle, however, is their morality. After all, proclaiming that "The Four are all equally bad" is a bit of a strecht. To start, all we know about Zindrach is that he was somewhat decent, to the point he would be disappointed with the current demons (which makes Ebira trying to resurrect him all the funnier). Is Brian a considerably ruthless person, who supports and defends a bunch of megalomaniacs? Yes. Can that also be said about a bunch of other characters? Also yes. And needless to say, both Aridon and Tregakh are absolute monsters with very little redeeming qualities. But then again, that can also be said about most characters!
The only thing that sets the Four apart from the other bastards is that they are considerably more powerful, knowleadgeable and difficult to kill than the rest. I guess, at the end, that main fault in the assumption of The Circle is that removing them will make things inherently better... which is not necessarily true. Sure, Aridon and Treghak are absolute monsters... but is Meya any better? Ornasion? The Bonelords? Or people like the Imperial Generals and Redfist? Those aren't going to suddenly start to behave just because the Four are gone. And what will do The Circle do then? Also "remove" these tirants? And so on and so forth? When will they say "enough"?
Or will they only kill The Four? And demons keep being exactly like they were, and the Undead go rampant, and the Empire continues with it's imperialism, and Gardon and Amari keep manipulating and deceiving. At the end of the day, killing the Four does not address the root issue that allows them to thrive in the first place. And assuming that "But I know the solution to all the problems!" the The Circle boasts... it's very Four-like, isn't it?
Imo, at the end of the day, I know where they are coming from: The Four have and unfair amount of power over everyone else, and they are not particularly good people. Henceforth, they need to lose that power... and their lives. And again, you can probably say the same about a lot of other people. If you only focus on removing "bad" people (bad from your point of view!), you will just become another genocidal megalomaniac.