Why the Smoke Monster was Created/Died by Lost Again

The Man in Black was special to the island in the same way that many of the Losties were (Jack, Locke, Desmond, Hurley). Mother originally intended him to perform Jacob's role but his desire to leave and growing corruption by the outside world made him unfit for leadership. Because of his "specialness", he wasn't destroyed when his brother threw him into light at the heart of the island.

But because his soul had become evil and corrupted after killing his mother (unlike Desmond and Jack, who entered the light with pure souls), the energy generated by the cork created the monster. The cork was blocking the release of hell or a place physically or metaphysically similar to it. I believe that the smoke monster represented a demonic, deathly force that had taken possession of the Man in Black's body and could appear as any other deceased person. This force represents all of the evil tendencies of mankind, it was a part of all the Lostie's lives but could only have a physical presence on the island. It was a somewhat different creature from the Man in Black but still retained some of his personality and desire to leave but also perpetuated the Man in Black's evil tendencies. The Man in Black was granted several powers by this energy (immortality, black smoke, persuasive speech, appearing as the dead) but was still trapped on the island as long as the cork was in place and the cork could not be removed as long as Jacob was alive. The monster also could not kill Jacob (or his candidates) or remove the cork himself and had to manipulate others to perform these tasks. Once Jacob was dead, the Man in Black lost the power to appear as the dead to keep him from appearing as Jacob. These were the only real rules he had to follow, everything else was "smoke" and mirrors. After Jacob was killed and Desmond removed the cork the same energy preventing Smokie from leaving and gave him his powers disappeared temporarily. I believe he was hoping to leave the island and then in desperation the Losties would replace the cork before the island was destroyed. After Smokie had left and the cork was replaced, his powers would return. Then he would spread death and destruction throughout the rest of the world. Or they would not replace the cork at all which would have ended the world anyways. But the Man in Black failed to anticipate the lengths that Jack and the Losties would go to to keep him from leaving and he also severely underestimated Kate.