Complex sentence: Tatsumi retorts that should he become tyrant's dragon body and go on a rampage, That she be the one to end him; both stating it'd be an honor to die by a brother in arms hand.
Split into simpler sentences: Tatsumi retorts that if he becomes a tyrant's dragon body and goes on a rampage then she should be the one to end him. Both state it would be an honor to die by a brother in arms hand.