Who's In The Coffin? (Spoiler-Free Theory)

What we know:
* The person in the coffin is someone we have previously met, per Darlton via podcast
* This person is known to both Jack and Kate
* The Oceanic 6 are harboring a Secret
* The Oceanic 6 were well-compensated
* There are outside forces, aka Matthew Abaddon, highly interested in the O6's ability to keep their traps shut
* Kate is incredulous at Jack's idea that she might have attended the funeral for said Man in Coffin, suggesting some sort of betrayal
* Kate is living much higher on the hog in her Volvo than we have ever seen previously
* Future Bearded Jack is 'sick of lying'. Kate has made a life-pursuit of lying, and has, apparently, no issue in the future with exalting this to her vocation, given her final conversation with Future Bearded Jack in TTLG and her Volvo
* Future Beginning Alcoholic Jack was a bit twitchy at the thought that Hurley might 'talk' when he was recommitted to Santa Rosa
Putting What We Know together, is it possible that someone who made it off The Island simply refused to keep the Secret? This person turned down the Oceanic settlement in order to be free to tell his story? Might that person have been murdered to keep him from spilling the beans about Craphole Island and its Secrets? There is apparently much at stake in spilling the Secret, or Future Jack would not be so conflicted--he'd just give it up already and campaign against recreational use of painkillers, wouldn't he?
It seems to me that there are those who would kill to see the Secret (and, by extension, themselves and their interests) protected. Perhaps our Man In The Coffin was a casualty.
Perhaps this person was an Oceanic 815 passenger who found rescue, yet refused to come in line with the story that the settlement recipients are relied upon to support? Or it could be an Island resident not associated with the Oceanic ticket counter who, nonetheless, will spill the Secret?
Remaining questions:
* What is the Secret the O6 have sworn to protect?
* Who would refuse to fall in line, at apparently great financial sacrifice in eschewing the settlement, and why?
* Why would the O6'rs alienate the Man In The Coffin? For Island crimes, or those (imagined or real) back in the real world? Or simply for refusing to keep the Secret?
* Why is spilling the Secret so dangerous that our O6'rs will become hired guns for the Benemy, retreat to psych wards or live in an alcohol and oxycodone induced haze in order not to tell?
Source: LostDocarzt