crackpot theory:
The monster ("security system") is to counteract the others, which is why they're on the opposite end of the island to the one the monster is active on. It's only on the one side because the button keeps it contained, which is why Walt said "the button is bad." When the monster went up to Locke and Eko, it scanned them (didn't see it scan Locke, but it's a fair assumption that it did), saw that they weren't Others, so it left them alone. Goodwin said they were looking for "good people", because the more non-Others they had, the less likely the monster would be to come after them, were the button not to be pressed.