Speculation and minor Dark Brotherhood spoilers ahead

As most of you probably know, the Ebony Blade is the Daedric Artifact of the Webspinner, Mephala. She is the Daedric Prince(ss) of betrayal, lies, and murder. After investigating rumors about one of Jarl Balgruuf's sons, the LDB finds the Whispering Door, behind which lies the Ebony Blade. The Blade is strengthened by the blood of deceit, and after killing a set number of people who trust/like you, the enchantment of Absorb Health on the Blade becomes stronger.

However, there is one thing a bit off about the enchantment and it deals with the quest Destroy the Dark Brotherhood. During the raid on the Falkreath complex of the Dark Brotherhood, the kill count on the weapon also counts the deaths of the Dark Brotherhood members towards the end goal. This doesn't seem to make sense at first. Why on Mundus would the members of the Dark Brotherhood count as people who trust/like me? The answer lies with the creation of the group itself and a bit of misunderstanding with Mephala.

While most in Skyrim (including the Dark Brotherhood) subscribe to the notion that the Brotherhood was formed by the Daedric Prince Sithis and worships/follows his word, there are those who believe that another Prince was behind the creation of the Dark Brotherhood. The reasoning behind this is simple: Sithis is the Void, an absence of life and sentience, so how could he create something? Instead, those people believe that it is Mephala who created the DB, just under Sithis' name (remember, she is the Prince of lies and murder). Creating a group devoted to her word that specialized in assassination, the pinnacle of murder and deceit is right up her alley, and confusing a few foolish mortals into believing her to be Sithis would be child's play.

Why do I mention this? It directly ties back to the Ebony Blade. There are two ways to look at this. Either the Blade itself is sentient and views the Dark Brotherhood as a fellow creation of Mephala, thereby counting them as allies, or Mephala saw you kill them and thought that maybe you were backstabbing your comrades. After all, why would you pick up the Ebony Blade (helping out Mephala) just to Destroy the Dark Brotherhood?

So, while this might just be a bit of developer oversight, I do like this theory, and the evidence appears to support it. Enjoy!