Mitya is convicted because everything the court can verify points to him, and because the only truth that would save him reaches the court only as the delirium of a sick man. The title of Book XII calls this a judicial error.
Against him stand the open door that Grigory swears he saw, the pestle found on the path, the money of hidden origin, the threats made in front of witnesses, the blood on his hands, the drunken letter written in a tavern and a confessed hatred. The prosecutor reads it all as a program and sets it out with the idea that the Karamazovs are capable of contemplating both abysses at the same time. The lawyer answers with the thesis that there is an overwhelming sum of facts against the defendant and not one that withstands criticism, and with a defense that, in the end, asks for mercy.
The two speeches explain the same crime with the same facts, and neither listens to what Mitya says about himself. The evidence that would come from Ivan, the money handed over by Smerdyakov, is discarded, because Ivan has a fever and speaks of the devil. The jurors, whom the ladies had judged incapable of such subtlety, answer guilty on every point, without the slightest leniency.
Mitya, for his part, shouts that he is not guilty of that blood; he wanted to kill, but did not kill. The court has categories for guilty and for innocent, and none for a man who is both things entirely.