Aerys II Targaryen

Aerys burned Rickard Stark alive in his own armor, strangled his son for protesting, and then sent to the Vale demanding two more boys to kill. Jon Arryn's refusal was the spark, but the powder had been Aerys's own work, laid down over years of escalating madness, wildfire, and terror.

He died with a Kingsguard sword in his back and a plan to burn his capital to the ground still unexecuted. Robert would say that killing Targaryens was less a policy than a public service, and where Aerys is concerned it is difficult to argue him down.