Complex sentence: William quickly deduces from Salvatore's speech that he had once been a member of a heretical sect and infers that Salvatore and Remigio may have been involved in the killings.
Split into simpler sentences: William quickly deduces from Salvatore's speech, that he had once been a member of a heretical sect and infers that Remigio likewise had been involved. He suspects that they may have been involved in the killings.