We knew Widmore was a former leader of the Others. He was on the island in 1954 when the time skippers, including his son Daniel, arrived to deal with Jughead. At that time, Eloise appeared to be in charge or the co-leader. Richard followed her lead more than Widmore's as time passed and their roles changed. The Others were clear followers of the word of Jacob. At some point, Eloise left the island (possibly to give birth to Daniel?), leaving Richard to scheme to find a replacement.
The Widmore-Linus rivalry may have started when Ben was recruited by Richard to join the Others. Widmore had ordered Ben to go kill Danielle and her child. When Ben got to his mission, he changed it. He kidnapped Alex and told an angry Widmore that Jacob had told him what to do. It was a lie. But the statement bolted Ben, even as a boy, up the Others leadership ranks. The finale coup de tat in the Others hierarchy was Ben's execution of the Dharma collective. The killing of his own father for the sake of the Island was held in great esteem by the Others. He had trumped Widmore. Shortly thereafter, Widmore was conned or "exiled" from the Island by Ben. The reason was unclear, but it could have been Widmore leaving the island and having another child, Penny. Widmore vowed to return after his banishment and reclaim his role on the Island. Another mystery was how did Widmore, a brash lad on the island in 1954, suddenly become a super-wealthy industrialist? Did he leave the island and take over all of Dharma's assets to become an instant millionaire?
For unknown reasons, Ben claimed that he was unable to kill Widmore even when given the opportunity, and that they both knew it. This relates to the Rules governing the two’s dispute, which he said Widmore "changed" upon his mercenary killing Ben's daughter Alex. But if the Rule was that family could not be killed, Alex was not Ben's real daughter (a loophole). As a response to this, Ben told Charles that he would find and kill his daughter, Penny.
Concurrently, Widmore was plotting to indirectly kill Ben by using Sun's anger against Ben to Widmore's advantage. It is unclear whether Ben's use of Sayid as an assassin was the counterbalance to Widmore's plan of killing Ben.
In 2007, Widmore finally returned to the Island in a submarine after claiming to have been invited by Jacob, who “convinced [him] of the error of [his] ways.” At this time, Ben's leadership role in the Others was gone. MIB/Flocke had taken control (under the threat of death). Ben became a weak follower, his vision of his future as Island leader, lost. It was under the vague promise of Flocke that if MIB could leave the island, Ben could have it. The only way that could happen was that if Ben killed Jacob, which he did.
Widmore claimed Jacob told him everything he needed to know to stop MIB from leaving the island. He brought Desmond back with him, as a last resort in case all of Jacob's Candidates died. Widmore was eventually shot to death by Ben in front of MIB/Flocke. Ben broke the Rule against killing Widmore directly. But it apparently had little consequence in the final end game of MIB's quest to leave the island.
What was the intense Widmore-Linus battle about? The power or control of the Island? But that was Jacob's role. The control of the Others? They both had leadership command, but it was not absolute. The island survivors were merely left over pawns in the game between Jacob and his brother. But the big build up of the Widmore-Linus "war" in Season 6 fizzled.
Was this blood feud supposed to symbolized something else? The Widmore-Linus dynamic of needing to return to the island in order to control it is the mirror opposite of the Jacob-MIB dynamic of the brothers wanting to leave the island at some point to move on.
Widmore's return to the island and his death had no impact on his life in the sideways realm. Widmore's death on the island did not trigger any awakening in the sideways realm, where he is the dutiful husband to all-knowing Eloise.
Ben's fall from leadership of the Others in some ways led to the death of Alex and Danielle. His acceptance of a role as second in command under Hurley's apparent island guardianship because he had no place to go. This second island term may have caused Ben to stay behind to make amends with Alex and Danielle in the sideways realm, but only after he was "awakened" by Desmond hitting him with a car.
It is debatable whether the Widmore-Linus story arc was an important aspect of the LOST mythology or merely filler.
