It’s all Jacob’s fault
Jacob’s actions have been motivated by guilt, that’s why his ‘protection’ of the island is different to his ‘mother’s’ — he knows he stuffed up and is trying to keep everything under control and contained while desperately searching for a way to end it all.
My thoughts on the light is that it is life, the essence of humanity. While protected and pure, it is good, the positive nature of humanity but contaminated it creates a ‘loop’ whereby it can absorb other traits of humanity, tainting it’s purity. The worse the intentions and actions of the people it comes into contact with, the more negative, evil it becomes.
The ‘mother’ protecting the island and the light was to stop any contamination. How long had she been there — who knows? I think she had had enough and bought Claudia’s ship to the island to find her replacement — who better to shape and mould than a newborn.
Jacob throws MIB into the cave with ‘bad’ intentions, he had been told it was worse than death and wanted to punish his brother severely. I think that smokie was created out of that — the first contamination of the light and water. Smokie is MIB’s soul and the way / portal whereby humanity can feedback back into the light, darkening it or lightening (depending on who smokie comes into contact with and their thoughts) So Jacob is now going ‘Crap, I just killed my brother, threw him into the place we were told not to go and this smoke stuff has come out. Ooops!’ Jacob then sees that part of his job to protect the island also involves keeping this monster / brother from entering the outside world, knowing that that is all his brother wants — to leave. If smokie feeds off people, it could then make sense that good, postive people will undo the negative, the evil. Is that one reason why Jacob brings people to the island, to try to show / feed smokie good? Unfortunately they come, fight, corrupt and destroy, feeding smokie with ill intentions, creating a monster of evil.
So Jacob has to keep smokie /MIB / his brother on the island against it’s will — imagine the consequences of all of humanity’s ill intentions fed into this creature. Not because the monster is evil incarnate but because we (and Jacob) have made it that way. Jacob also wants to die — he has had enough and keeps bringing people to the island also with the hope of finding a replacement. MIB can’t leave while someone is protecting, or able to protect the island — all candidates must die.
One reason for this way of thinking is Locke’s meeting with smokie in season 1 — it was beautiful, Locke only had love and gratitude for the island then, smokie absorbs and reflects what it is given.
Jacob’s actions contaminated the pure light and have turned it evil, with each attempt to prove MIB wrong and show that people are innately good, he inadvertently provides more evil to be fed off, making the situation worse and worse. Jacob seems to have done what he does out of guilt for one massive stuff up and is trying to stop it completely destroying the world.