Official synopsis of Supernatural 4:16: On the Head of A Pin
"SOMEONE'S KILLING ANGELS -- Someone found Lucifer's sword and is using it to kill angels. Castiel (Misha Collins) and Uriel (guest star Robert Wisdom) have captured Alastair (guest star Christopher Heyerdahl) and ask Dean (Jensen Ackles) to use the torturing skills he learned in Hell to extract information from him so they can stop the murders. Sam (Jared Padalecki) is concerned Dean can't handle the job but Dean agrees to do it. However, when Alastair reveals some shocking information, Dean's world is shattered." Source SpoilerTV.
Who is the angel killing douchebag? I think it could be Anna. Why? Anna knows the ways of sending the angels away in I Know What You Did Last Summer. In Heaven & Hell, when asked if there is any weapon that can be used against an angel, she said "Nothing we could get to. Not right now." She is a rebel, and she hates her past life. I think while she is able to take her grace back, she isn't willing to go back. Or may be somehow some demons extract the information from her about this weapon (assuming it's Lucifer's sword).
Suspect #2 is Lillith. That's part of her job description. If she knows about which seals to break in order to free Lucifer, then perhaps she has the knowledge about his sword and uses it against the angels.
What shocking information would shatter Dean's world? It has to be something about Sam and his relationship with Ruby. Perhaps this is the 'something' that leads to the brother vs brother situation.
3 words: I CANNOT WAIT!