It’s just that from my modern perspective, there is actually something to be said for being loyal to your friends above and beyond loyalty to a state or an ideal – it is, in fact, this sort of eternal conflict that comes up over and over again under many regimes...

See, I think I've read too much Roman stuff, because the "... I would hope I'd have the nerve to betray my country" response strikes me as deeply unsettling, except in cases of tyranny. Sometimes I worry that I've gone native. Then I read Juvenal or Horace and realize that, no, that's not going to happen.

Actually, patron/client relationships are a lot more fluid than feudal relationships -- you don't have anywhere near the same kind of responsibilities, and clients can have ties to more than one patron, who may be in conflict with each other. People have a lot of negotiating space in picking and choosing among their patrons -- and in acquiring new patrons, too. (The one exception is freedmen, who are pretty much tied to their former masters.) But I think you're right that to Dante it looked like Brutus had betrayed his lord, indeed, his king, and within Dante's frame of reference that might be pretty much unforgivable.