In wotlk his holding onto the light is what allowed him to resist Arthas. Then when Arthas is slain bolvar takes up the helm, not necessarily an action against the light? Tirion was going to wear the helm, but bolvar insists he should. Given they both view one wearing the helm as a necessity I dont really think this would be the point where the light would completely turn its back on whoever wears the helm (is the light that which allowed Arthas to keep some sense of control after becoming LK?)
What then happens when the helm is removed? Can bolvar still call on the light? Are we approaching a scenario where his return to the light saves us / anduin? The lack of other high ranking paladins presence in the shadowlands disturbs the logical flow the story should be going imo, id think we'd have more anti death focused individuals with us, but so far we are meeting them already here; uther and mograine.