That's kind of the point though; while the Crusader is good at forcing back demonic forces, he's pretty much willing to use any tool to do it. He might well be able to close all those Hellholes if he was allowed to operate unchecked--but the price would be way, way too high.Well he hasn't gotten to the whole "enslaving the world to the Dark Gods" thing yet. And a fair number of people think he never will, that the forces of the Abyss are endless. So it's setting an evil (the Crusader) to contain an evil (the Diabolist) and stopping either of them from mustering enough power to threaten the right-thinking folks of the Empire.
Honestly, I suspect the only reason he's tolerated is there's just not enough forces working for the Great Gold Dragon and the Priestess to handle the problem.