This is another situation where our actions, and our motivations for those actions, aren't dealt with very well.
When I first saw the Oathbreaker show up in the panel demo, I was curious how it would differ for paladin's of evil deities. I guess that was foolish on my part.
Currently, you don't select any deity as a Paladin and the two subclasses are goody-two-shoes. Devotion is based of the pre-5e Lawful Good Paladin. Of the Ancient is "there is beauty in the world and I must protect it" that focus on good and fighting off darkness and despair. Neither would work for a Paladin following an evil deity. I'm not quite sure what you were expecting there.
If you want to role-play a paladin who follow an evil deity you have to go Oathbreaker. Maybe at release they will have added Conquest or some homebrew evil Paladin subclass since even Vengeance is about fighting off evil.
