I think for Dorian's case, it is also finding out that his father, whom he had once thought as a virtuous man (He must have been, most of the time, or Dorian would not have turned out the way he did), would do something so against everything he once preached and practiced.
That for anyone would be hard to swallow. I mean imagine a parental figure you looked up to all your life, who taught you about morality and rights from wrong, suddenly commit a crime the very same figure once denounced. (ala Surprise! Your dad, who is a pillar of the community, beat up your step sister when she was in elementary school.)
Even if you forgive that person, the relationship will be awkward for a long time to come, maybe forever. For Dorian, it is even worse. First his father, then his mentor. Each falling to Tevinter's worst vices which they both used to despise. Perhaps Dorian sees the possibility of himself in his elders, and wondered more than once if that too is where he would one day fall.