The Board of Immigration Appeals has found that a person is subject to the persecutor bar if they assisted or aided in the persecution of any person and the persecutor was motivated to harm the person on account of a protected ground, even if the assisting individual was not so motivated. Thus, because Alvarado stood guard while his superiors tortured someone on account of his political opinion, he is subject to the persecutor bar, even though he was not personally motivated to harm the person on account of his political opinion.
The full text of Matter of Alvarado can be found here: