Quote Originally Posted by Fyraltari View Post
They're not free-willed. They can't help but do the morally right/wrong thing. Unlike the undead.
They actually are free willed - there are cases of angels turning evil and demons turning good in DnD.

This is common enough that the rules of creature types address it:
Evil Subtype
A subtype usually applied only to outsiders native to the evil-aligned Outer Planes. Evil outsiders are also called fiends. Most creatures that have this subtype also have evil alignments; however, if their alignments change, they still retain the subtype. Any effect that depends on alignment affects a creature with this subtype as if the creature has an evil alignment, no matter what its alignment actually is. The creature also suffers effects according to its actual alignment. A creature with the evil subtype overcomes damage reduction as if its natural weapons and any weapons it wields were evil-aligned (see Damage Reduction, above).
http://www.d20srd.org/srd/typesSubtypes.htm#evilSubtype

So yes you can literally be made of evil and still be a good person.