Or it burned him as a warning not to break one of the Ten Commandants in this case. I don't think it was about "wicked fairy" influences, because 1] I don't think Mab or the Winter Court are evil. 2] I don't think the mantle is evil in of itself.. 3] Perhaps more importantly previously the Sword didn't burn him, the mantle was still there, I doubt if it were evil in of itself the influence would stop. Think of the mantle as a tool, it enables a human Knight to do what he or she has to do. Can it be used for evil in the wrong vessel? Yes, it can, but that doesn't make it evil in of itself of wicked fairies.
The Swords do not act to prevent simple human error -- even Sin -- when driven by simple human passions like fear, love, anger, pain, etc. Sometimes the Knights will do that, offering compassion and wisdom.
The Angelic powers of the Swords only come into play when they need to neutralize Supernatural impacts.
The primary task of the Swords is to counter the Denarians, but they help whenever the Supernatural is threatening to take away humans' free will.
Harry felt pain because he was "acting under the influence," not from his own free will; or, possibly, because (under the WK Mantle)
he himself was a "Supernatural" influence.
My bet, though, is that Lasciel was the Guilty Party here; that she's still got some sort of hooks into Harry.
