Everyone always believes they are doing the right thing.
Even as they murder, kidnap, govern, rape, steal, and trespass- as they archate.
If they truly believed they were doing wrong, they would stop. You aren't going to convince them that what they want to do is wrong-- except in exceptional cases. They have excuses and justifications for what they want to do handy and rehearsed.
So... what to do about it? (Short of killing them all, I mean.)
The best strategy I've found is to live by the Zero Aggression Principle. You may have other ideas.