Some Red God worshipper received a vision in the flames that Sam would be the one to acquire the horn of Joramun, that could bring down the Wall. So, the worshipper hires a Faceless Man to kill Sam and claim the horn. But all the worshipper saw was that Sam was a member of the Night's Watch. So J'aqen H'ghar is sent to go after Sam, and does so by landing in the King's Landing dungeons, expecting to be sent to the Wall as a recruit. However, we know what happens to him, and he gets side-tracked a bit by Arya and all that fun stuff, but eventually departs to continue his mission. Of course, Sam acquires the horn north of the Wall (assuming that horn Jon found with the dragonglass daggers is the true horn), and is then sent to become a Maester at Oldtown. J'aqen learns of this, and follows Sam there. J'aqen then makes the deal with Pate to acquire the Citadel master key, and then steals Pate's identity to get close to Sam. That's where we are now. So, J'aqen kills Sam (sad), and delivers the horn to the buyer. The worshipper wants the horn because he wants the Wall down, so he can begin the apocalypse. The Red God can have his climactic battle with the Great Other that way, and restore light to the world. So, he's nuts. But in doing so, the Wall does come down, and **** hits the fan.
I don't have a theory for who the worshipper could be (obviously a wealthy person capable of paying a Faceless Man), but there are any number of candidates. But my theory kind of rests on the idea that it's a Red God follower.