This fight was long overdue. How dare Theodore come into his territory and threaten to end his guest's life? If that wasn't a blatant insult, Aldric didn't know what else it was. Theodore has been pushing his buttons over the years and this time, he was not going to let him be.
What would Islinda think of him if he does nothing and let his asshole brother go like that? That he wasn't capable enough and the reputation he's built over the years would take a blow. No, Theodore had it coming and Aldric was not going easy on him. If he didn't make an example of him now, Aldric feared what he might do to Islinda in the future.
But then came Andre, always the conciliator, the peaceful brother, and choosing to stay on neutral ground. However, Aldric didn't care if he wrecked his castle, he had enough money to rebuild the damages as long as he dealt with Theodore and Islinda was safe.