The Sith use the same terminology as the Jedi but that doesn’t follow that they believe themselves to be evil.
Batman doesn’t think he is a villain when he doesn’t dismiss the term “Dark Knight”.
There may be honour in controlling and using the not immediately seen, the hidden, the nocturnal side of the Force.
When Luke says “There is good in you, I have felt it”, Vader could easily retort, “Yes I am Good, I am serving the people by giving them stability and order, there is good in you too, join me”.
He as good as offers this to Luke in ESB.