Merlin is ?.

[AtlantisForever]

well in Season9 EP1 daniel said that Merlin was Murdan and in Season10 EP6 daniel/DR Wier meet's Moros in that holigram room but that holigram was female... but some are you are sayin he's an old man.. i no in before i sleep moros is an old man but why a female?

[JedI Master of the Gate]

I think he should be Moros twin brother. When they left Atlantis they traveled back to their home-galaxy, and discovered the Ori, Moros was killed Merlin escaped (The Ori couldn't find out where he went, or where they came from) Mulin Ascended try to convince the others to attack the Ori, they wouldn't so he took matters in to his own hands.

[IcyNeko]

well in Season9 EP1 daniel said that Merlin was Murdan and in Season10 EP6 daniel/DR Wier meet's Moros in that holigram room but that holigram was female... but some are you are sayin he's an old man.. i no in before i sleep moros is an old man but why a female?

The spoilers mention them seeing Morgan LeFay in the hologram room, not Moros. Where did you read that it was MOROS?

Anyway, It was partially Moros's fault that the Ori are here. It was in his lab that we found the long range communications device.

... could it be that Moros was secretly a spy for the Ori among the Alterrans and that, after ascension he realized the error of his ways and tried to fix it by creating an anti-ori weapon? but since the ancients knew of his treachery before, they watched him carefully (sent Morgan after him)? This would be an interesting twist.. and would explain why Moros was so pissy towards Weir.

[McSwift]

I do give props to whoever made this post.

I thought it was good and seemed correct, but the spoilers pointed otherwise.

[JedI Master of the Gate]

... could it be that Moros was secretly a spy for the Ori among the Alterrans and that, after ascension he realized the error of his ways and tried to fix it by creating an anti-ori weapon? but since the ancients knew of his treachery before, they watched him carefully (sent Morgan after him)? This would be an interesting twist.. and would explain why Moros was so pissy towards Weir.

I just Had an Evil Thought, what if Merlin is an Ori? What if Arthur wasn't a devote Christin, but a devote follower of Origin? And What if Merlin wasn't trying to distroy the Ori, but the Ancients?

[Primus Commander Woden]

would be an awesome twist...Daniel's faith once again tested hehehe

[Kirath]

i'm sticking with my original theory. My theory is that Merlin was an Atlantian who after returning to the milky Way ascended. After he ascended he learned of the Ori, their hatred of the Ancients, their forcing humans to worship, draining the life energies of their followers to increase their powers and their goal to one day destroy the Altera. The war with the Wraith changed him. He didn't believe you could just sit back and allow evil to grow stronger and that sometimes a peaceful solution just wasn't an option. He realized that even if the Ori didn't know that human life flourished in the milky Way, eventually they would come looking for the Ancients and then both they and the humans of our galaxy would suffer. In order to get around the rules of the collective he retook corporeal form. From Weir's visit in Before I Sleep he knew that on our world a race of explorers skilled in gate travel would one day emerge. He knew of our battles against the goauld and our aiding other races and decided earth was as good a planet as any. he built his anti-Ori weapon and hid it leaving clues in our history that would one day lead us to the Avalon cave. He planted numerous tablets like the one Vala found on earth and other planets written in a Ancient dialect that few except for Daniel would be able to translate. Daniel and his history with ascension and the Ancients may have come up in Weir's explanation of how the atlantis team rediscovered the city. His hiding the device and morgans watching him probably has to do with the ancients none interference rule. The idea being that if we find the weapon and use it, they themselves aren't involved. Its all about choice. we choose to use the weapon or not to, but it has to be our decision. Just like Daniel going to their galaxy in Avalon. They may have shielded us from the Ori's perception is one thing but if we choose to do visit their galaxy in some form that different. if the Merlin tried to use the weapon they may have tried to stop him fearing he may start the ascended war they have been trying to avoid. Morgan might not stop or hinder Daniel but she may test him to make sure he can be trusted with a technology potentially dangerous to the Ancients themselves and able to use it wisely.
As for how the humans on camelot got there. He may not have brought them there. He may have simply built his library and planted his legend into their history.