where is all of this "sithis is nothingness" coming from, sithis is a huge somthing, very very very much somthing, he is atleast half of the creation of nirn, he is the embodyment of change, and chaos, which of course puts him closer to the deadra than the aedra(divines) but from what i have read he is in no way shape or form "nothing"
The members of the Dark Brotherhood all give very personal descriptions of Sithis. I'd make a reference here to a real world group that adheres to the same doom and gloom concept but I can't think of any.
The ENTIRE IDEA of Sithis is that everything was perfect until things happened creating an automatic imbalance.
It's more nuanced then that.
The book http://www.imperial-library.info/mwbooks/sithis.shtml holds what appears to be a Dunmer vision of Sithis. The book considers the myriad of possibilities, the ebb and flow of ideas at the start of the Aurbis ideal.
As also described in the Altmeri myth, this state of constant change was caused by Sithis. The Altmer however have another view of Sithis and as you described consider that the stasis before Sithis sundered the nothing was perfect.
This difference is one of philosophical the schisms between Altmer and Chimer/Dunmer. It's the spiritual predecessor of the Darkbrotherhood, the Morag Tong who first cultivated Sithis as a basis for a thuggish murder cult.
With aid of Mephala they reasoned that not only change and death as caused by Sithis are good, but also that the Daedric element of action and thus actively causing death was a good thing. When you compare it to real world religions it's easy to see how this underlaying philosophical idea can be transformed into the "Sithis wants me to kill you/Jezus loves you" mentality of the Dark Brotherhood.
edit:
The last paragraph is based on http://www.imperial-library.info/obscure_text/chaos.shtml.
I'm also taking great joy in calling the Dark Brotherhood degenerate. Especially considering the whole Dark God with a Dark Mother Mary their child the Dark Brotherhood, vibe that surrounds the DB.
Obviously, you've not read enough.
Thats' never made anybody listen to an argument.