The Smoke Monster existed on The Island before the events that took place in "Across The Sea", and continues to exist now, after "The End".
We see evidence that The Smoke Monster exists on The Island prior to Claudia's arrival at least three times in "Across The Sea".
1) The Mother alone could not have destroyed the well and killed off the entire group of men by herself, especially not in a few hours. She likely either summoned The Smoke Monster in the same way that Ben did, or is actually The Smoke Monster, herself. I'm going to assume the former. The Smoke Monster was also interested in setting up The Man In Black to fail in his quest to leave The Island in order to incite his rage, which leads to him killing the false mother and being tossed into The Light by Jacob.
2) Claudia appears to The Boy in Black. This could have been evidence that he had a similar power to Hurley, but I feel that Claudia was The Smoke Monster appearing to The Boy in Black, attempting to sway him toward "The Dark Side".
3) "How do you know all of this??", "I'm special', Mother!"
- The Smoke Monster gave The Man in Black additional insights into the mysteries of The Island.
- This combination (his mother giving him the info) eventually made him obsess over The Light, and put him in an easily manipulatable position. The Smoke Monster just had to tap him a few times to get his attention.
Jack is the new Smoke Monster.
- After Jack turns The Light light back on, as Desmond is being pulled back up by Hurley and Ben, The Man in Black's theme music begins playing, as if to say, "You know what happens when someone's down here and they're not immune to electromagnetism". It's a very, very ominous scene with Jack laughing (somewhat maniacally) lying in the pool of water and light.
- With all of this talk of Giacchino's music being it's own character on the show, you must take what it says into consideration.
- Jack appears in the same location as The Man in Black's body after The Smoke Monster emerged from the cave. I feel that it is also significant that they are facing opposite directions, as seen right.
- The Island wasn't done with human Jack until he had seen the plane fly overhead. It is what he had set out to do since his eyes opened in the first scene of the series.
We did not see a new Smoke Monster emerge from the cave from Hurley and Ben's perspective. I think this may have been a choice by the production team to let us speculate a bit more on the meaning of the final moments, or maybe Hurley & Ben just left. Perhaps instead of a "Smoke Monster", Jack became the opposite, a "Light Monster". To Hurley and Ben it just appeared as if The Light was reemerging from the cave. Maybe that's part of the significance of Jack & The MiB facing opposite directions (this was most definitely a choice made by the production team). The false mother claims that if ANYONE goes down there (save Desmond, later on as we find out) that they have access to life, death, and rebirth. Jack was down there when The Light was on. Something big happened to him, and with help and time he was able to create the alternate reality with the other survivors. From here on out, I'm referring to whatever comes out of The Source, whether good or evil as "The Smoke Monster".
The Smoke Monster takes on personality characteristics of the human form that it inhabits.
- "Don't tell me what I can't do!" - He does this kind of stuff all the time.
Jack as The Smoke Monster, Hurley as The Guardian.
- The MiB's final thoughts were a huge desire to leave The Island, and great hatred for The Island itself and all of its inhabitants. These thoughts are brought to life as The Smoke Monster that we all know.
- Jack's intentions were "good" when he died. He was trying to protect The Island, save the people that wanted to leave, etc.
- Hurley's job was probably very easy after Jack killed the MiB.
- "You were a great number two" "You were a great number one" - Yeah, Hurley & Ben had it easy after Jack set it up for them to knock 'em down. They probably even had Jack's help afterward.
There must always be a Guardian. There must always be a Smoke Monster.
- The false mother knew this. All of "Across The Sea" was a 30 year con by the mother to set up the next era on The Island
- "Thank You" - why else would she thank The MiB for killing her unless she knew that she had done all of what she needed to do, and that she would have to die in order to do it? She had passed on her power to Jacob, and set up The MiB to be in a position where Jacob would throw him into The Light and create the next Smoke Monster. Until both of these things were done, she would not be able to move on, and neither would the previous Smoke Monster.
- It took her so long because she always wanted The Boy in Black to become The Guardian. He was the more "human" one of the two boys. Jacob would have been a much "better" stoic, silent, judging Smoke Monster.
- "I brought you here because I made a mistake", "Jacob didn't know what he was talking about".
- Jacob doesn't know all of the ultimate secrets of The Island. He is human and foul-able. He still thinks he made that mistake long ago (creating The Smoke Monster), but he did exactly what he was supposed to do.
I think that ultimately, the era of The Island that we know is so evil and twisted because there was no free will involved in the creation of it. Neither Jacob, The Man in Black, or likely their false mother had a choice in becoming who they were. Yes, there are powerful and dangerous things there, but The Island doesn't have to be a dangerous place. Since the people were stuck there, trapped there by forces beyond their control it became like a prison. For nearly everyone else on The Island that actually wanted to be there, they were interested in the power that The Island held and little else. Eventually it became a life and death struggle for everyone involved because they had no choice and no control over what was happening to them, and no way to leave.
Now, since everyone still on The Island remains there by choice, with "good" intentions, they can shape The Island to a different end. Essentially, the scale has been tipped back the other way.
I hope you enjoyed, and I'd love to hear anyone's thoughts.