Complex sentence: At last, he comes to Agat's castle, fights his way inside but the ronin has never killed an innocent, so the sword is not powerful enough to destroy the demon.
Split into simpler sentences: At last, he comes to Agat's castle, fights his way inside. But when he confronts Agat, he has a dilemma: since the sword has never killed an innocent, it is not powerful enough to destroy the demon.