We're talking about the kind of hypocrite who doesn't know he's a hypocrite. He wants to get witches, that's a genuine desire, but unbeknownst to himself, his deeper, stronger motivations for all this are career, self-aggrandizement, and self-love. So he gets sloppy about his work, harrassing people with insufficient evidence, because emotionally, nabbing people and being the perfect gpodly hero matters more than fairness. He's also an egotist who assumes he makes no mistakes. He also may have bitterness about other things that he vents in his work.
This goes on in the world all the time. Saying someone's "sincere" isn't necessarily saying very much. Brainwash yourself to believe whatever you do to people is holy, and there you have a form of sincerity, because he has successfully convinced himself.
The louder Trask's voice is, the more he feels the need to convince himself again. He's trying to make his beliefs true through sheer volume.
