I don't actually believe this. It's not really so much a headcanon as a dark and twisted AU what if.
What if Uther was right? What if magic really did corrupt people?
Morgana, Mordred, Gilli, even Merlin: the more they used magic, the more extreme and obsessive their actions got. A lot of them even seemed to lose it a bit beyond the end. It was most noticeable in Morgana, but even Merlin started showing signs of paranoia (understandable paranoia, but still), obsessiveness over Arthur's safety (You want to leave Gwaine to die? GWAINE?), etc. It's less noticeable in him because his goals aren't selfish like the others, but he's definitely not the same Merlin he was.
And the reason this theory is wrong: Of course he changed! He'd been making life or death decisions in secret for ten years while under constant threat of execution!
So not actually true. But it would have been an interesting twist if it were . . . I mean, the show is from Merlin's POV, more or less. Everyone's views on magic are either those who use it (biased) or those following Uther's crazed Purge (biased). We never get an objective study of how using magic changes you. Even Anhora was willing to let thousands starve to prove a point to one man.
Like I said, I don't actually believe this. Just wanted to see what kind of reaction it'd get.
