@OT: I don't think its ever been confirmed who is the Night Mother or her name, just hints into the role's origins in that Morag Tong and its connections with Mephala.
Also as far as satisfaction, its worth noting almost always there is a median of some sort to interprete Sithis's will, and if you run under the assumption the DB is just another variant to the Morag Tong with the same patron, then it shouldnt be too surprising my the toll in death is always.
If taken to the more literal, I don't think Sithis actually cares so much over one soul or two, but that violent change is occuring and piece by gradual piece the Void claims all once more. But like, I also just dont think Sithis is as invested into their whims as they personify him to be(Sithis isn't a unique deity if their's, argonians and Hist also worship him) just the same that your average DB assassin isn't a full faithful much like their Morag cousins. Not at all to say traditions arent honored and the devout lack sway.
Sithis is not a real God in the same sense that Stendarr is.
Sithis is very much a god, being the very personification of Padomay(the primordial chaos) but not of the et'ada(since obviously he predates them)
The problem isnt so much his divinity, something even Daedric princes attest to(even denoting some of their oldest and most powerful kin closer to Sithis) but in so far that his worship is either unambiguous much like another stamp on history.. a matter of fact nobody bothers to spend two thoughts on, or direct worship is shunned without any of the potential rewards of his spurned spawn(i.e Daedra are very invested into affairs, he's even less so than the Aedra, which isnt much)
The simple fact is, even if you just ignore the hairline differences between Primordial and Personification, by the end of it all Creator Entities in TES arent as revered when you got more proximal deities(Lorkhan, Auriel/Akotash) or those straight up responsible for your race or culture(Malacath, Azura).
Also, when you start to apply mortal belief into deity alterations, at a certain point that becomes as minimal to different interpretations of creation, similarities or not you cant *change* the origin. When you start talking about Primordial forces or mere universal concepts like death, change, absense what have yeah.. there really isn't much to shape especially when whatever entity
is there could be more alien or omnipotent than those we are accustomed to. Its like trying to bend a blank slate of absolutes. Sure its easy to draw a face on it or have vague guesses on intents, but bend it will not, and a false lie is all that you will carve
Also just to nip this in the butt.
Sithis =/= Lorkhan
Padomay ≥ Sithis > Lorkhan