WARNING: This entire question contains spoilers for A Dance With Dragons.
After the battle at the Wall, the Free Folk are given the chance to 'bend-the-knee' and be pardoned in order to be spared, including Rattleshirt.
A short while later Mance Rayder is burned upon the Pyre by Melisandre and Stannis.
We later find out that it is not actually Mance, but Rattleshirt who gets burned.
After the burning, Jon Snow is introduced to a subdued-by-Melisandre's-magical-bracelet-thingy RattleMance in Stannis' chambers. At which point Jon has no idea the true identity of RattleMance.
A little bit later on, in the yard Jon defeats some of his men in training, after which RattleMance turns up and challenges Jon to a duel. But Jon says something really curious:
“The big crow can peck the little crows,” growled a voice behind him, “but has he belly enough to fight a man?”
Rattleshirt was leaning against a wall. A coarse stubble covered his sunken cheeks, and thin brown hair was blowing across his little yellow eyes.
“You flatter yourself,” Jon said.
“Aye, but I’d flatten you.”
“Stannis burned the wrong man.”
“No.” The wildling grinned at him through a mouth of brown and broken teeth. “He burned the man he had to burn, for all the world to see. We all do what we have to do, Snow. Even kings.”
-A Song of Ice and Fire: A Dance With Dragons, Part One - Dreams and Dust, Chapter Twenty-Eight (Jon).
Why does Jon say that?
Is it because:
- He knows Rattleshirt is actually Mance?
- He thinks they should have burned Rattleshirt? (Most probable)
- Something else?