While helping Narfi of Ivarstead discover the fate of his sister Reyda, I noticed something quite disturbing. The quest reward for helping Narfi is always three random alchemy ingredients. The three ingredients given to me this time were human flesh, human heart, and nightshade.
Those are the three ingredients needed, along with a skeleton and a circle of candles, for the Black Sacrament. I felt this was disturbing as the game never gives any clues as to who would have Narfi killed or why, in the Dark Brotherhood quest line. With the ingredients given to me, it would suggest that Narfi may have been planning on using the Black Sacrament for himself.
A hole in this theory is that Narfi begs for his life during the DB side quest “Kill a hapless beggar.”