The belief structure is completely meaningless. The facts are the facts.
And a 9th level caster can cast
commune and ask Pelor "is it bad to do things to this corpe?" If he says "yeah bro, that's totally defiling and like rude" then that is also a fact.
Knowing something for a fact and believing something without a doubt are
functionally the same thing. They are not the same thing but they act like the same thing.
I don't
know that the light is going out in the refrigerator when I close the door, I just believe it very strongly. I don't need to close myself in the fridge to see for myself. And knowing for sure won't change how I act.
Not unless Pelor reaches down and Consecrates/Hallows/Blesses/makes into magic items the corpses of all his followers.
Just because an item does not have the "holy" special quality does not mean it's okay to mutilate. Things can be a sin even if they're mundane. Most gods have some mundane restrictions and dogma.
I'm not talking about any homebrew. I am talking about the rules.
(You were actually referring to some setting you were writing.)
Predatory, sure, but malicious? I'd like a source on that.
Do you have a source that they're not?

In all seriousness, most intelligent undead are typically written as being uniformly evil. There are the typical exceptions, but that's standard of D&D, as it just
needs those tortured good guy vampires, and liches, and drow.
Or, let me put it this way: Is it wrong to eat a cow? No?
Vegetarians say yes. And cows also aren't sentient creatures. And they're seldom alive when you begin to eat them.
Then is not an old and high-level vampire as far from a human as a human is from a cow? From its perspective, it's simply eating a herd animal, a lower creature that's alive to be food.
That's what
they believe yes. But they are EVIL so I think they might have some bias.
Seen it in a few undead entries.
But I'm already 20-minutes late for bed so digging through a dozen monster books from five editions of the games seems... unproductive at best. And the Hypertext SRD is pretty fluff light so that's of no use.
No, you can't. They're Evil because they're Evil. Evil is completely arbitrary. There is no "taint" involved, it's just that the cosmic forces declare them to be evil and therefore they are arbitrarily evil, dead stop, the end.
Also, you forgot ghosts.
Ghosts are intelligent, so no.
And I think we've given so big justifications over this thread, so I wouldn't say it's completely arbitrary.