This would mean that alignment is not a part of you as a person. You might be the most virtuous person in the world, but still end up in the Hells because you aren't omniscient and didn't know some of your actions would have bad consequences. This seems pretty fucked up and ripe for Avellone style deconstruction.
D&D ethics is primarily intentionalist, not consequentialist, with that cosmic dimension. I.e. if you were acting out of Good motives and in tune with the Cosmic Good, but evil nevertheless resulted (probably due to interference by agents of Cosmic Evil), then that would not doom your soul (nor cause a paladin to fall).
There's a parallel that might make it easier to understand. Consider a modern-day religious zealot who believes that (1) his religion's law is God's Law and (2) he, or his religious leader, has a divinely inspired understanding of it. Such a zealot would strive to act in accordance with God's Law in every situation, regardless of imperfect or downright Satanic human laws.
D&D ethics is like that, except that there really is a universally acknowledged Cosmic Law (although being what it is, it cannot be fully expressed in any human terms; even the most just laws enacted by the wisest and most benevolent of kings are at best a pale reflection of it), and there are ways to determine if a being or act is in accordance with it or not. That's frankly terrifying -- your religious zealot isn't just convinced he's right, he's
objectively and demonstrably right. In those circumstances, it would be immoral to
not go to any lengths needed to act in accordance with Cosmic Law -- it is the proverbial, final, ultimate Greater Good after all!