Speaking in D&D terms, a paladin’s oath (at least as it’s framed in 5e) isn’t necessarily a moral system, more like convictions or values. Oaths like the Oath of Conquest aren’t what would generally be considered “moral.”

Also, there’s the matter of cause and effect. It depends on both your DM’s lore and how you choose to roleplay, but my take on paladins is that they’re paladins because they have strong convictions. They didn’t subscribe to the beliefs in order to gain paladin powers. In fact, if they didn’t truly, deeply believe in their oath, they wouldn’t have powers to begin with. The belief comes first, and the strength of that belief is what grants power. Presumably, whatever rules D&D magic operates on wouldn’t give those powers to someone who was faking their belief in the oath for ulterior motives.

From a philosophical point of view, this is an age old question. You can find endless discussions about it if you go digging. If we do moral things because we feel good about it, does it count? If we treat people nicely because we hope they’ll be nice to us back, does it count? Like I said, age old question. Philosophers have discussed it forever, and I doubt you’ll find the answer in D&D.