They fought for the Imperial army, which means they fought for you, General. You were bedfellows with the men who killed your father, who abducted Flayn, who destroyed Remire Village. Whatever excuses you may have had for not raising your sword against them from the start, that doesn't change the fact that you joined hands with them to slaughter all who opposed the Empire.

[Seteth's lips twist in disgust at that. Hubert's letter had implied the partnership wasn't in good faith, but even so--the idea that Byleth could stomach working with such monsters at all, yet not believe him..... It truly does illustrate how little she must think of him, doesn't it?]

Interrogate their mages, if you want to learn their secrets. Or search their stronghold, if it still stands. It can only be the fruits of their sins that inspired Rhea's madness--she wouldn't answer my questions. Perhaps you'll have better luck.