Here is the line:
" A man in dark armor and a pale pink cloak spotted with blood stepped up to Robb. " Jaime Lannister sends his regards. " He thrust his longsword through her son's heart, and twisted."
Catelyn was watching...so it seems more likely to be one of Roose's men. Otherwise it would have read "Roose Bolton in dark armor...."
Roose had been sitting next to Catelyn but had excused himself for the privy earlier.
The fighting was everywhere, the bedding ceremony was going on, but there were still plenty of individuals left. Catelyn, Robb, Smalljon, Dacey Mormont, Lord Walder, Ser Wendel Manderly, Edwyn Frey and a host of others were still in the main hall. The crossbow bolts that took Robb and others down were from above the room.