The Others are Jacob's Experiment. Everyone is just a threat by Sir Moxie

The initial group of people in Jacob's experiment were the survivors of the Black Rock. The present-day Others are their descendents as well as a number of other people whom Jacob felt worthy to be a part of the new civilization. This is where Jacob's 'lists' come into play. When new people arrive on the island, the smoke monster judges their character and, if they are 'approved', then they get to be a part of the civilization. If they are not, then they are either sequestered (like Rousseau), or if considered dangerous enough, they are killed.

This explains the behavior of The Others where they will not include any unapproved outsiders in their civilization, whether it was Rousseau and her team or the flight 815 passengers. It's why they wouldn't share info with the 815 people and why they wouldn't try to help them when they arrived. It's why The Others and the Dharma people had to keep strictly defined borders. They were instructed by Jacob to include only the people on the list in their civilization. Everyone else would be considered a threat to the success of his project. Charles Widmore was kicked off for bringing too much outside influence to the island.

The reason for the loss of fertility and also the reason for Richard's lack of interest in this fertility issue is a result of Jacob's reaction to the "incident". Once science moves forward and mankind displays an ability to expolit the island the way the Dharma team did, Jacob needs to take the exclusion/inclusion process to a more extreme level. Eventually the Purge will get rid of all outsiders and, in addition, Jacob stops allowing children to be born. The only people who are allowed into the experiment will hanceforth be members specifically approved by Jacob. Children of members are unpredictable. Instead of furthering the civilation with new children, he decides to venture out into the world to groom future members for himself. Thus the interaction with the Losties that we saw in the Season 5 finale. He was grooming a new set of members over the years and, when they were ready, they were all brought together to the island.

Now... the loophole... Jacob and MIB cannot kill each other. But Jacob has placed all of his faith in humanity. Like any God through the ages, the one thing that can kill a God is a lack of followers/believers. In the end, Jacob's civilization started to question him to the point of asking to see him, following Locke to get proof of his existence. Their faith was dwindling. And one of his most faithful stabbed him. That was MIB's loophole. He had Jacob killed by his own product.

Jacob acted as though everyone had a choice. He told Ben that he had a choice before he stabbed him. But it was all talk. Jacob was manipulating Ben the same way Ben manipulated other people. Essentially the only choice that Ben had was the choice that Jacob left him with. After all of his terrible experiences, the only choice that Ben had was to kill Jacob. And so, in effect, Jacob was the cause of his own death. Thus... the loophole. So, is the MIB the bad guy? Or is Jacob? Or neither...

I'd be interested in hearing what anyone else thinks of this.... to add more to the concept or to disprove it somehow? Fun!