I would go so far as to say MoldyWart trusted Snape a bit more than other Death Eaters, because he saw some of the same emotions in Snape that he had experienced when he was that age, and believed he would know if Snape changed his mind and heart.I take Dumbledore at his word, when he said (repeatedly) that he trusted Snape, and despite appearances, I believe there was a solid reason underneath it