Just came to my mind : did whe see the monster in the 70's ? it wasn't shown in the 50's and if in Dharmatimes it was not on the loose, the first time it became active was when the French Team arrived. So where was it at that point ? Imprisoned ? Was it sort of a deity of justice to the others as Ben claims ? and why did it start attacking later on. It is also noteworty that as of now it never attacked an Other (there was that one time with Juliet, but i think it just wanted Kate. And Brams Team where no Others)
It was built by Horace (unless that was somekind of supernatural con) in Dharmatimes which leaves only a (relativly) small timewindow for it to be Inhabited.
a) Either MIB was imprisoned in the cabin (circle of ash) and Ben thought of it just as a creapy place to play his prank. In this case most likely Hurly opend the prison when he moonwalked away from it. But then remains the question about the monster because it was clearly active before that point, so either there are a number of monsters out there or the prison was brocken before the French Team arrived and it just choose to live there(doubtfull).
b) Jacob actually lived there at some point and the circle should keep MIB out. In that case why did he move out of the foot and why move back in later on ? This also leaves the question why the circle would be broken.
Also in generaly don't you think "thats enough, you had your fun" is a strange reaction from someone who thought the place was empty and than it becomes alive ? I dont think Ben worked with smoky at that point, which leaves Jacob or a third yet unknown party to spook them out. And i think Ben workes for this party, because "I always have a plan" and "I lied. Thats what i do.".