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Jamie Lannister aka Kingslayer
In Robert's Rebellion, the army led by Tywin Lannister came to Kings-landing from Castle Rock, but they looted the city of Kingsland. The angry Mad king orders Jamie Lannister, who was in the kings-guard, to bring Tywin's head. Instead, Jamie puts his honor aside and kills the Mad King with his own sword.
Jamie Lannister is sitting on the Iron Throne with a bloody sword in his hand when Ned Stark comes to Kings-landing on horseback. Jamie was 17 years old then.
Seeing that sight, Ned Stark gets a little nervous.
But Jamie Lannister says, "Don't worry, Stark, your friend Robert Baratheon was just there to keep the chair warm, and I doubt the throne is such a comfortable seat."
17 years later, Ned Stark reminds Robert Baratheon of that incident. In the matter of Vale's control.