The most disturbing season finale starts with the flashback of the Black Rock arrival as Jacob remembers it, doomed to a loophole that repeats over and over another man swears this will be the last as he'll find a way to kill him.
Throughout the episode, different flashbacks shows Jacob taking contact with the castaways through several points in their lives: the funeral of Sawyer's parents, Kate's first theft, Jack's first surgery, the first day Jin & Sun spent as husband and wife, the last time Sayid ever saw Nadia and so on, the only one Jacob doesn't contact is a younger Juliet...
And yet is older Juliet the hero of this story, as every attempt to change history fails once the drill actually hits the magnetic core of the island is our sweet, brave Juliet, the most capable woman Ben could find to fix the pregnancy issue and the only woman Jim "Sawyer" LaFleur ever committed to the one who finally detonates the hydrigen bomb leaving us hanging in our seats for a shocking next season.
For it was not a plane crash what they were meant to stop, it was not the incident, not even the Dharma Initiative but rather the murder of "He who will save us all" the answer Illana, Bram and the rest finally receive from Ricard Alpert as they show him the content of the box they were carrying: the body of our beloved John Locke as dead as the day Jack took the Ajira 316 flight. Meanwhile, in the shadow of the statue, the man who we thought was John Locke has finally killed Jacob just as he swore he would ...when the Black Rock arrived.