While I sort of agree with the Kilgharrah being a troll theory floating out there, I also feel like that's giving him too much credit. Yes, he manipulated Merlin into doing a lot of things. But to say that the prophecy was a trick, and that Arthur wasn't supposed to be the Once and Future King? I don't know. That prophecy had existed for far longer than the time of the Great Purge and Kilgharrah's imprisonment under Camelot. Perhaps he just sought to break their destiny and seal Camelot's fate.