As always, LB, you offer both the most straight-forward explanation and most reasonable link to the mythology of Castlevania.
I always felt that Death was a very subtly structured being in the series once it came to defining his character. The massacre of innocent lives, the dominion of vampires upon the earth; all seemed little more to him than the elements of a much larger picture only he/it can perceive on a level of consciousness that no earthly being (vampires included) could ever really appreciate.
On the other hand, if we are to consider that the dark lord is the being directly opposite to God than it wouldn't be a stretch to imagine the harbinger of the end to all existence as holding such a powerful source of destruction in, to some degree, a measure of respect or even esteem. Perhaps in some way Death recognizes that the active force of darkness that offsets the light in our world is necessary and thus works to help aid in its progression purely out of instinct. And maybe... Just maybe mind you, Death as a divine but dark being knows that the dark lord is fated to lose. Best to maintain the balance as much as possible in the one-hundred year stretch darkness is allotted while it's possible.