Robin and Ace have similar arcs, but in this moment we see a bit of a difference. For all intents and purposes, Ace spends most of Marineford as a pawn on a chessboard. The Government doesn't treat him as a person as much as a symbol for the age of piracy that Roger wrought. While Robin actively wanted to die, all his life Ace has questioned his right to be alive.
This is him accepting his position as pawn. So long as he's on this execution platform he'll never be Portgas D Ace. Instead he's the Pirate King's Son, and all that implies.
It's important that when Ace does die, it's after being freed from the platform. He was able to pass as himself, in his brother's arms, surrounded by the family that would literally go to war on his behalf.









