I've been playing with the idea of separating 'Undead' from 'Unlife'.
Undead are not inherently evil and are your pretty run of the mill ghosts, spirits stuck somehow, maybe good spell casters that have extended their existence in a Lich or something. Unlife are actively trying to destroy the living world and inherently 'evil'. I'd treat them a bit like an element (Fire, water, earth, wind). 'Created' undead is kinda would depend on who created it, how, and why. If you had a friendly Lich creating skeletons, they'd be undead constructs. If you had a powerful entity of unlife creating them, they'd emanate that (fear, dread, foreboding, whatever) and be more dangerous. This all would have some impact on certain spells, holy items/weapons, etc.