Abbadon means "guardian of the abyss." Just as Richard is/was Jacob's mediator between Jacob and others to try to get them to do what he wants them to do, so Abbadon was a mediator for the MiB. Abbadon's job was to get people to make decisions that would cause them to act out MiB's will.
We know that: Ben killed Abbadon. Abbadon worked for Widmore. Ben hates Widmore. It can be theorized that Abbadon also works for MiB. Therefore, Widmore and MiB may be in cahoots.
Not impossible, but EXTREMELY improbable. Widmore and the MiB are at war with one another, both want the other dead, and the MiB is not at all concerned when Ben kills Widmore. And if Widmore and the MiB are enemies, it is equally unlikely that Abaddon is in league with MiB unless he is a double-agent.
After the freighter blows up Jacob meets with Widmore, if Matthew worked for MIB then im sure Jacob would have told Widmore
Abaddon was one of the Hostiles.
He was "exiled" alongside Widmore.
A hospital orderly turned agent
He tells Locke to go on walkabout based on a tip off from Widmore/Hawking/The Nemesis, and from then on he is in Widmore's Pocket.
He actually did go on a walkabout. He says it to all the sick people he meets (as genuine advice), and he didn't actually know that the plane was going to crash. Somehow Widmore finds out that locke was put on the island on Abbadon's advice, which led to him going to the island. After that, he's required to do tasks for widmore.
One of Jacob's "Body Guards"
Abaddon was working with Jacob to get Locke to the Island and after the Kahana exploded and Jacob appeared to Widmore he sent Matthew to help him.
The problem with this is that Abaddon was never seen on the Island.
Fair enough but prior to Ajira 316 niether was Bram nor Ilana but they both did work for Jacob
Matthew Abaddon had a similar role to Richard in that he worked directly for Jacob as a messenger, only Abaddon's role was more involved with the recruitment of candidates and being an influence for candidates to end up on or return to the island should it difficult without intervention. We've seen him in at least four situations in which he is shown to be doing this, posing as an orderly to Locke, interrogating Hurley at Santa Rosa, recruiting part of the Kahana team, and helping Locke reunite the Oceanic 6. It is clear from this that he is one of the few people, along with Widmore, that are aware of the feud between Jacob and The Man in Black. Therefore he is one of these few people that are still truly working for Jacob, as opposed to Ben and his people, who are being manipulated by The Man in Black. This explains why Ben sees Abaddon as a threat, having been manipulated into thinking that he is the enemy and ultimately murders him. Abaddon must have encountered the Island at some point, but it seems, given his role as a recruiter/intervener, he spends most of his time off-island.
And he knows this because Charles Widmore told him. Widmore remembers when Locke first appeared on the island looking for Richard Alpert back in 1954...and knows that Locke is the future leader of the others.
Abaddon is the one who paid Malkin to get Claire on the plane. He pretended he wanted to adopt Aaron, knowing Claire would end up on the island.
Abaddon told Naomi that their were no survivors, so that the rest of the team on the Kahana would really believe what they were going to do. Since the mercenary team only were used as killers, they didn't found this important. Naomi was thus the leader and had to tell the lie further so that rest of the team would believe them. Since Abaddon worked for Widmore, Abaddon was aware of Widmore's plane with faking Oceanic Flight 815.