Dont worry its not a time theory.
It looks like after a certain amount of people managed to get off the island, there are still many deaths going around for some reason. First it was the man in the coffin, then the "are they still alive?" remark, and then Sayid going around killing for Ben.
I think the flash forwards will show what ultimatly led Jack to decide he has to go back.
This theory doesnt have much to hold to, and may be shattered in the next ep, but still:
So Sayid works for Ben, who wants to off people that may have ways to get back to the Island. Ben does that because he doesnt want the Island to be found, or he just doesnt want the Oceanic 6 to ever get back to the Island, because that may jepordize its secrecy, for some reason. We know Sayid is usually rational, so maybe there is something on the Island that should better be kept secret, if Sayid is willing to sell his soul and work for Ben. Then Sayid kills Else...
Hurly wants to go back to the Island, after seeing Charlie that tells him he needs to.
Then he confrons Abbadon, who I believe is the Economist. After Else died he starts to get scared, exposing himself like this, but as soon as people come he disappears, knowing he might be next. Hurly meets Jack, who tells him he's thinking about growing a beard, which may be too obvious of a hint, but still.
Abaddon is soon found by Sayid or some other assasin and killed.
Then there's Jack, all bearded, himself now wanting to go back. He goes the funeral of Abaddon, possibly one of the main people who knew how to reach the Island, and we know the rest.
Does that make any sense?

I believe that the flash forwards are going in reverse order. In other words, the first Jack flash forward is the one farthest in time from them leaving the island. Then comes the Hurley flash forward. Then the Sayid flash forward and finally this last flash forward with Kate being the closest so far to the time they left the island.

There are various reasons for me believing this. The state of Jack's beard being the most telling point. The Kate Flash forward comes before the Hurley flash forward because as one of the Oceanic 6, she has one of the most recognizable faces in the country. But by the time Hurley gets caught in the car chase, he has to remind everyone he is a member of the Oceanic 6. Also when Jack goes to see Hurley, he is just starting his beard and in Kate's flash forward he is very clean cut.

The only flash forward that is hard to place is Sayid's. But I think it must have come before Hurley's because the man Sayid killed still recognized the Oceanic 6 which, I believe, places it closer to Kate's flash forward in time.

I don't think the coffin is for Abaddon who, unless he was cut in half, would be far too tall to fit into the coffin Jack was standing over.

I'm not sure the guy Sayid killed recognized him, or at least not instantly. When Sayid told him though he seemed alert but not sure what to expect.

Abbadon maybe's not the man in the coffin, but I still think its someone Sayid killed.

I'm not sure the guy Sayid killed recognized him, or at least not instantly. When Sayid told him though he seemed alert but not sure what to expect.

Abbadon maybe's not the man in the coffin, but I still think its someone Sayid killed.

I don't know if the guy necessarily recognized Sayid, but I'm sure he recognized the name Oceanic 6 and what it meant well enough. He wasn't just alert -- he was scared. By the time Jack is in full beard, the Oceanic 6 is barely mentioned at all. The funeral worker does not recognize him and even when Jack's name is mentioned in the pharmacy, only one person says he's a hero and even that is probably due to the TV report about his heroic work saving the boy and his mother from the car crash. The TV report itself makes no mention of him being a member of the Oceanic 6.

I do think it is possible that Sayid has killed the guy in the casket. Jack might even have some idea that Sayid is responsible for the person's death. Also, given Sayid's difficulty killing people he cares about, it would be an extremely ironic twist if the last names on Ben's list for Sayid happened to be members of the Oceanic 6.