...most of the time, people - specifically, people, not just demons who do things because of their nature - sin because they have some higher priority that overrides doing what they know is the right thing. A need for money, or love, or revenge, for example. Sometimes it's as simple as being miserable and needing to make others miserable to feel less alone.

It's very easy for humans or merpeople to justify to themselves the evil that they do. All they need is a reason.