Why is creating undead Evil?

[Xgya]

That's the thing though: playing devil's advocate to Gary Gigax' original intent, in the tabletop game of D&D where the rules of morality are objective, an [Evil] Vampire should never behave good, and a [Good] Archon should never behave evil. Following an Archon is always good because Archons should never be presented as evil to players. If the GM makes a morally questionable Archon, he's literally playing the game wrong. A good character would view the very idea of resorting to an [Evil] spell as unthinkable, and should be played to do everything in its power to avoid it. Failing that enough, it should become [Evil] and start behaving accordingly. Such a system is self-coherent, but it requires the GM and players play by its rules, not the real world's.

I'll say that Good is a concept that can have gray areas. In fact, Eberron shows it directly as such, by giving examples such as two Paladins being Lawful Good and generally acting as such, yet managing in opposing warfare to Smite Evil EACH OTHER, because they think the other side is definitely Evil.

This generally falls upon morally gray areas, whereas a world with rules defining a clear line dividing Evil from Good HAS to pick a side, and automatically sees the other side as Evil.
Give them the Trolley Problem, and you've got one Paladin saying the other side is Evil for pulling the lever, thus having their actions cause the death of someone, while the other Paladin saying the first side is Evil, for NOT pulling the lever, which would save more lives and refusing to save someone in danger is against their own code of ethics.