Since he first appeared on the show, Matthew Abbadon has been one of the most intriguing plot lines on LOST. There is so much about him that is unknown, yet he still seems to be a major cog in the overall LOST story.
First thing to take into consideration is his name. We all know that names on LOST are not created by chance, and that many of them have direct meanings associated to the actual character. The name Abbadon translates to satan, anti-christ or the destroyer in biblical materials. It really doesn't matter which one it is because from those choices we can understand he is one bad dude.
In his first 2 appearances in the show we see him trying to pry information from Hurley and orchestrating the Freighter mission. From this we can divulge that he is more than likely working for Widmore as the head of finding the island. After these appearances he didn't seem too mysterious but then we saw the scene with John Locke in the hospital. By watching this scene it would seem that Mr. Abbadon has some sort of power to see the future, as evidenced by him advising Locke to take a walkabout. Is it possible he could see that John Locke would crash on the island just like Claire's psychic knew that she would also crash? What's even more interesting is when Abbadon states that he and Locke will see each other again, that Locke will "owe him one." What does that mean exactly? What does Abbadon look to gain from Locke crashing on the island and eventually seeing Abbadon once again?
Taking all of this into perspective, this is my theory:
Matthew Abbadon is employed to find the island for Widmore. The way he attempts to find a way to the island is by using his knowledge of the future. He is to Widmore what Ms. Hawking is to Ben. Ms. Hawking uses powers that are yet to be described to find the island for Ben while Abbadon uses psychic abilities. Sometime after the year 2000, Matthew Abbadon has premonitions of how John Locke reaches the island, the time he spends there and how and when he gets off of the island. Once Locke is off the island Abbadon will come in contact with him and will somehow use John Locke as a means to get himself and Widmore back to the island. By providing a way to get back to the island John will be repaying what he "owed" to Abbadon. Will John have to die in order for this to happen? If anyone would kill John Locke, it would have to be someone nicknamed the "Anti-Christ." Thoughts, comments?