I used to think that self-righteousness was a symptom of hypocrisy. Y’know, point the finger at someone else “worse” than you, in order to distract from your own sins and shortcomings.
Now I think it is the opposite. Kind, open-hearted, good, decent people start out pure and fair. Then they care about something, and become righteous about it. Then they become self-righteous. And self-righteousness is a cancer. It eats away at your eyes and your heart. You stop seeing things you used to see. You stop understanding people you used to understand. You stop seeing how other people really are.
And then, and only then, after years of self-righteousness, do you become (what critics call) a “hypocrite”. Someone who has lost the very thing they loved, who has unwittingly undermined the very ideal they had cherished.
First, you are innocent, with pure motives, and you care about justice and righteousness. Then your righteous indignation morphs into self-righteous anger. Then you embrace an ideology that increasingly consists of self-righteousness. Then that ideology will start to fill your heart (bit by bit) with scorn and contempt for others. Then (ever so slowly) it will make you blind, first to others, and then to yourself. Eventually, without even realizing it, you will become part of the problem that you cared so much about in the first place.
So it isn’t that “self-righteousness” is bad because it’s annoying or offensive to others (who are probably equally annoying).
Rather, self-righteousness is evil because it’s so God damn dangerous to you very own self.