I actually know the answer to this question. Its covered in the old chaos books.
Krell is not under Kremmler's direct control from a necromancy standpoint, he is a servant of chaos. Chaos gods can curse champions to fight in undeath and that is Krells fate.
Think of him like he is to evil to die, rather than a corpse that acts as a filter for dark magic. (those of us who have read too much old fluff).