At some point Arthas stops using the spells and attacks of a Paladin and begins using Death Knight abilities.
Is this a conscious decision on Arthas's part or at some point did The Light forsake him?
I ask because The Light certainly doesn't seem opposed to killing in general. Paladins can fight one another and use the Light to do so. The Light heals, but so do several Death Knight skills Arthas has so healing is not exclusively pure and good.