The Inquisition assumed Trevelyan was helping out of good character and a sense of duty. The Inquisitor liked it that way, but beneath all the Herald and "hero", lay a cold-blooded murderer and an ambitious ruler. The Inquisition just stoked the flames out of embers long dead and cold, and the mage wouldn't rest untill all Thedas is consumed by fire and re-forged in a new form.