I'm finding it hard to believe simply getting someone else to kill Jacob is the "loophole". Seems like an insanely easy loophole to find, if that's the case. Plus, that sounds more like a condition to be met, not a loophole...loopholes are the rare exception. "Somebody other than you must kill Jacob" doesn't sound like a rare exception, because any of the other 6 billion people on the planet could kill Jacob lol 
I really have zero idea what the loophole might be that allowed Jacob to be killed.
I think that the loophole is more complicated than the man-in-black just needing someone else to kill Jacob. Because that task in and of itself is VERY complicated. The way I see it, the "loophole" is the method by which the man-in-black can successfully get past Jacob's "protectors" and have him killed, without having to actually kill Jacob himself. Like someone else mentioned before, like a game of chess. But where the opponent's pawns are used to kill their own King. In which case, the "loophole" as we now know, was revealed by using John Locke.
First, up to this point we've been led to believe that there are two individuals on the island involved in Jacob's protection. Each one of them with a unique role. There is Jacob's personal liaison = Richard, who Jacob made immortal, presumably so that he can protect Jacob forever. And then there is the leader of the army that does Jacob's bidding = first Widmore, and then Ben. As times changes, the members of the army grow old and die, so there is constantly a need for new people to be a part of it. It's therefore a revolving system of protection, where Jacob only needs to count on one person, Richard, while the rest are expendable leaders and new bodies. So, to infiltrate that system of protection required someone who could not only get the trust of both Richard and Ben, but also be able to manipulate both of them so that they not only brought the man-in-black to Jacob, but also killed Jacob for him.
So, this "someone" is the loophole. The loophole had to be an off-island arrival who when arriving on the island came to the realization that they need to not only stay there, but protect it (and by extension, Jacob). Which would help to align him to those already doing that job (Richard and Ben). Additionally, he had to not only impress them with his committment to the island, but also be in a position where, for some reason, Richard and Ben would look up to him and start treating him as their new leader, or as someone "special".
The man-in-black did this by first giving Locke the ability to walk again. Which won him Locke's allegiance to the island, and by extension a "false" allegiance to Jacob. By orchestrating the time travel where Locke was able to go back in time to visit Richard, this set up Richard to influence Ben later on when Locke reappeared in the future. So, now Locke seemed like his own meaningful leader with a history on the island, rather than just an outsider or a stranger. Since he had regained the ability to walk, this impressed Ben, who had cancer. And since he had not aged a day in 50 years, and spoke of things that had yet to happen, this impressed Richard, who was the only other inhabitant of the island who did not age.
So, with that set of manipulation in place (the man-in-black using Locke, Richard, and Ben) the man-in-black had to then orchestrate it so that Locke would be killed by Ben out of jealousy, yet also create a circumstance by which Ben would bring Locke's body back to the island, so that the man-in-black could use his body as a vehicle. From there, he would be in a position to use that person's body and identity to further exert his influence over both Ben and Richard. Ben would follow Locke, because Ben had killed him and, being human himself, obviously would see that Locke was powerful enough to have been brought back to life. And Richard would follow Locke, because Richard had never seen anyone brought back to life before either, so that meant that Locke was obviously more special than he was as well.
In between this, the man-in-black used living Locke, as well as the death of Ben's daughter and Jacob's silence to Ben, to corrupt Ben's faith in Jacob. And he used Locke to mislead Richard.
So, when the time came, and the circumstances were right, in Locke's body he was easily able to convince Richard to take him to Jacob, and he was able to convince Ben to kill Jacob for him.
The only hitch here I think is why Jacob never warned Richard about John Locke? If Jacob had been regularly communicating with Richard, wouldn't Richard have mentioned this guy who had been traveling through time? Why wouldn't Jacob have instructed Richard not to believe in anyone who comes back to life, or at least warned Richard that Jacob had a nemesis on the island who was constantly trying to kill him?
Which perhaps is where Jacob's loophole comes in? Or is that part of the arrangement between Jacob and his nemesis, that neither of them can speak to anyone about the other's existence?
