My current theory is not that there's something morally wrong with siding with Hoshido specifically, but rather, that there's something morally wrong with siding against Nohr.
Basically, here's my idea: Garon is evil and cruel, but Marx is actually a decent person (and Garon's son) and finally, his father has gone far enough that he's realized his only choice is to reform Nohr. Violently. Starting with assasinating his own father.
He asks his adopted sibling Kamui for help, and in fact Kamui becomes absolutely crucial to the plan, where Marx has no hope of succeeding without Kamui's help. Unfortunately the plan isn't ready to be implemented yet, and before it can, Nohr and Hoshido go to war, with Nohr having a massive advantage and being set to butcher the Hoshido. And Marx isn't yet in a position to stop it yet.
And so, Kamui finds him/herself presented with a choice. Betray their siblings in Nohr, the people who raised them, the only family they've really ever known, ending any hope of Marx reforming the kingdom and going back on their promise to Marx, dooming the Nohr to continue their misguided ways, eventually being forced to fight against and perhaps kill the only people who ever showed him love...
Or side against the Hoshido, the people who are in the right in the current conflict, plus his own blood kin, and in the process personally lead, for a bit at least, a campaign of terror and depravity against an innocent and loving people who were begging him to help them, possibly fighting against and kill his blood family who just wanted to welcome him back...