The attempt to unify the various story aspects of LOST is a difficult chore.
One cannot be positive about anything.
As Oscar Wilde wrote, “All art is at once surface and symbol. Those who go beneath the surface do so at their peril.”
Exactly. What was the true peril in LOST?
What was the one fear that bound together everyone?
It may be a basic human inner terror: dying alone.
The composite feature of any of the main characters were that they were basically loners wandering through life with little or no true friendships. Some say that it is not how you perceive your own life, but your life will be judged by those who attend your funeral.
Human beings have a tribal instinct to belong to a family, a community, kindred spirits. But during one's life, those connections can get lost - - - trampled by the pressures of work, obligations, derailed by alcohol, drugs or quests for power, or tortured relationships including rejection.
That is a heavy dose of DOOM that people think is shadowing them throughout their lives.
If we examine what was below the surface of the island, we find two things. First, we find the ancient Egyptian temple complex, with a drawing of the smoke monster sitting across from Anubis, the god of the underworld. Second, we find the mysterious light force which is said to bring life, death and rebirth through supernatural powers which includes moving both time and space. Despite what is shown on the surface of the island, below is the clear symbolism of death and the after life. And the smoke monster is clearly depicted as part of this underworld realm.
Attached to the subsurface of the island are the roots of the plants, including the banyon trees which some believe have magical powers to ward off evil because spirits reside in their roots. Juliet and Kate were saved from the attacking smoke monster by hiding in the tree roots. What also is tied to the surface of the island? We would learn from Michael that the whispers are trapped spirits who cannot move on in death. Michael was one of those trapped spirits when he spoke to Hurley.
So we could conclude that the island itself is symbolic border between the living and the spirit world. We can also conclude that the smoke monster is a form of a spirit that is trapped on the island. As a spirit, it has magical abilities to change matter and form, to probe the minds and memories of human beings, and to destroy or kill. In all natural systems, there is a balance in order for the system to sustain itself. If the smoke monster is a evil, dark force, then the light force represents the counterbalance of good. It would have its own representative shape or smoke monster form on the island - - - which probably is symbolic of the island guardian such as Jacob.
Jacob being an energy being, a spirit, can explain why he could give Alpert the gift of life on the island because he was connected to the life spirit who can give life and rebirth. Thus, it is fair to assume that there are more than one smoke monster on the island. This could explain why Rousseau's reanimated dead crew members came after her, to turn her into another smoke creature. It could also explain why there was an obsession with new born children. Evil spirits who are trapped or chained to the island because of their evil past may believe that taking a new born, free from sin (pure goodness), absorbing that soul could be the key to releasing their bonds to the island underworld.
We have an island filled with symbols of death and the rituals of the underworld. We have an island inhabited by immortals and spirits. Indeed, the island is thus a magical place not fully of Earth.
If spirits are energy beings, the uncontrolled release of the EM pulse such as Desmond's failure to input the containment numbers causes the spirits to surge into time and space to attach themselves to human beings or to draw them (shipwreck them) on the island. So we can have the 815 plane crash survivors being live, human beings living in a spiritual realm that seems, on the surface, just another Pacific island.
There has to be some sort of unwritten bargain at play. The trapped spirits need to have humans come to the island for their own redemptive purposes, so their chains can be released so their souls can move on. But redemption is not what happens to any of the main characters on LOST. In fact, no one really has a defining revelation and life changing redemption on the island. There was no more compass that judged good or evil in their hearts. So what could the island spirits give the castaways that was so important, so valuable, that it could redeem them?
Since the spirits are dead, they had experienced the human frailty of dying alone. The island visitors have not gone through that end life moment. The spirit world would give them one last chance to find true connections with other human beings to avoid the fate of the whispers. Friendship, which includes affection, love, respect, trust and deep memories, was the passport for the 815 survivors to reach the sideways church, which was symbolic of their own group funeral.
When Christian said that "they" created the sideways universe, he was probably mistaken. It was the released spirits who created the supernatural alternative sideways world to hold departed souls in a state of ignorant limbo until everyone in the group was ready to "move on." The freed island spirits created the sideways world as their last penance before they themselves could move on. When know MIB could shape shift forms, so we can assume other spirits can too. And using the memories of the human visitors, the spirits and the island magic could create a realistic alternative world. And this could explain why it was slightly different, because a person's memories contain both factual recollection of past events as well as a person's dreams. So that may be why the spirit sideways world had Jack married to Juliet.
The bargain was simple: if the trapped island spirits could change human beings to be good, then they could be released from their island purgatory, and thus helping the humans from their inglorious fates of dying alone (and being unable to move on, like trapped spirits). The theme of redemption had little to do with the main characters, but it was the stake for the invisible characters, the island spirits.
This bargain unites two major elements of the series: life and death. How one lives their life is important, but it is also how one lives in death that is just as important. It answers the question of why people were brought to the island (to release trapped spirits). It answers the question why MIB was frustrated (most humans became corrupt-evil and turned into more whispers trapped on the island like himself). It also answers why an unlikely bunch of diverse people from Flight 815 could do something no other visitors could accomplish - - - because they truly changed their lonely paths and made strong friendships and bonds with unlikely people which enhanced the goodness in the island's life force. The reward for this bounty was the release of the whispers, who in turn rewarded the castaways with something they could only dream about: dying together, and not alone.