2 “And it was morning; and they themselves,” that is, those who brought Jesus, “went not into the judgment hall,” to wit, into that part of the house which Pilate occupied, supposing it to be Caiaphas' house. And then in explanation of the reason why they went not into the judgment hall, he says, “lest they should be defiled; but that they might eat the passover.” For it was the commencement of the days of unleavened bread: on which they accounted it defilement to enter the abode of one of another nation. Impious blindness! Would they, forsooth, be defiled by a stranger's abode, and not be defiled by their own wickedness? They were afraid of being defiled by the pretorium of a foreign judge, and had no fear of defilement from the blood of an innocent brother: not to say more than this meanwhile, which was enough to fix guilt on the conscience of the wicked. For the additional fact, that it was the Lord who was led to death by their impiety, and the giver of life that was on the way to be slain, may be charged, not to their conscience, but to their ignorance.
Source: Tractates on the Gospel of John (New Advent)