Board Thread:General Discussion/@comment-80.167.139.52-20130723183001/@comment-7953253-20130724060058

Probably Astrid is the antagonist. She betrayed you and everything the Dark Brotherhood stands for. She assumes the mantle of the leader of the Dark Brotherhood. The kind of authority that rightfully belongs to the Night Mother and the Listener.

When the Dragonborn (Listener), Cicero (Keeper) and the Night Mother arrived in the DB sanctuary in Skyrim, she didn't like the idea of having her authority being dismissed. So she wanted to get rid of them, Listener and Keeper, and ignore the Night Mother's ways (Telepathic communication with Listener). This is not the way of the Dark Brotherhood, and so she had to die for it.