If he pulled out of the treaty with everyone even though only one individual betrayed him, I'd say it's most likely that either that peaceful demon lord really did lay a trap and his wife paid for it, or that he at the very least was too mentally fragile for any treaty to last with him. Otherwise, he'd have either not lashed out at those cities or he would have at least continued the treaty with the other nations.
Then again, Japanese media tends to be flavored by their cultural, extreme belief in group guilt. Something many mangaka try to fight against.