He saw his wife shot in the womb that was carrying their son two meters in front of him. He felt the still warm blood seeping through the open cuts in the flesh of the woman he had dared to love. He saw his mother's throat cut after begging for her life. He heard his army slaughtered all around him and in the distance.
Robb Stark screamed as three arrows pierced his chest and a knife slammed into his heart. He felt the life drain from his falling body. He saw all this and sighed for the last time. After that, there was nothing.
Twyin Lannister ordered the Red Wedding, but Walder Frey got blood on his hands and on the house. The old lord of the Twins laughed at the cruel comments about the fate of the bodies of the King in the North, his queen, his mother and his direwolf.
For a moment he cursed that he had not come up with the idea of cutting off the Young Wolf's head and sewing on Gray Wind's in its place. Walder Frey thought it much more coherent to throw the lifeless body of the "young king" overboard, once the invaders from the Iron Islands destroyed Winterfell.
It made sense, it just wasn't as funny as people's rumours. The old man laughed miserably at not having such a brilliant idea. Not that it mattered, after all, House Stark no longer existed and the North no longer had a king to call its own.