A smotherfucker is someone who gains sexual gratification by means of smothering or choking thier partner either during or piror to intercourse. The person being smothered is most usually not aware of the smotherfucker's intentions until it is too late.
Dan: "The face she made was priceless. So blue and terrified! Hehehehe!"
Mary: "Dan, you're such a smotherfucker!"
Much like the term fucked, this implies being screwed, in that shit is about to hit the fan for you. Motherfucked implies that the situation is far more grave.