Why the smoke escaping was bad:
Because the smoke was made of the energy deep within the island that belonged solely ON the island as the island was the only place on the planet that could support and protect it. If that energy escaped into the real world it would have destroyed it. The smoke was twinned with the personality of a person who desperately wanted to journey off of the island, at any cost, even if it meant taking whatever the energy he had bonded to was. In the end, the personality bonded to the smoke wanted to leave despite the damage the smoke energy would have caused to the outside world.