Why is creating undead Evil?

[hamishspence]

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.

Actually, I've been rereading 3.5's Eberron rulebooks = and none of them mention anything like that. BoVD has a relative morality option - but it specifically states that it is not the standard.

Two paladins might be stupid enough to try to use Smite Evil on one another without Detecting Evil first, but by RAW, neither will get the bonus damage, even in Eberron.

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.

That might have been true in 1e, but it's not true in 3.5. The MM specifically states that exceptions to "Always Good" and "Always Evil" creatures exist, but also that they are either unique, or rare. Some adventures (like the Sertruous adventure in Elder Evils) include an Evil celestial, for example.