Understand what happens when you find yourself in a relationship with a self-righteous person. You get preached at and constantly disapproved of, this should be obvious. But there's more. You get subjected to propaganda, you get to watch hypocrisy in action, too.
Everything bad about people and having anything to do with them stems from some sort of insecurity. So much of evil comes from the need to compensate, this is not the kind of person you want to be around all the time.
People have a way of hiding their more nefarious, greedy, self-aggrandizing motives behind good-sounding principles. You may find yourself dragged into truly bad situations, doing bad things because of your association with the sanctimonious.