Tywin Lannister

Tywin Lannister sat out the entire rebellion until its outcome was no longer in doubt, then arrived at King's Landing under a flag of friendship and sacked it. The city fell not to a siege but to a lie, and the brutality that followed — the murder of the Targaryen children among it — was the price of the crown Robert wore.

Robert accepted the alliance, married the daughter, and never fully reckoned with the bill. Tywin's grandchildren would sit the Iron Throne after him, which had been the point from the beginning. He is the coldest arithmetic in this story: a man who joined at exactly the right moment and was paid in dynasty.