I was thinking.... what if the flashback characters were working with or for Dharma?Then that would explain why i think they wanted to get payback.
For example:
Hurley-lying to his best friend
Sawyer-for conning that girl
Jack-letting his dad down
Kate-her mom (for killing her dad)
Charlie-letting his brother down with their band
Ecko-(his brother)for not shooting that man and sending him away
Michael-(maybe walt's other dad)maybe changed his mind about walt and wanted him back
Sayid-Torturing that man
Sun-not marrying that man
Jin-(suns mom,if she was alive) killing suns father
Bernard & Rose-the healer that couldn't heal her cancer