Original: Meanwhile , Fire Lord Sozin , who killed the previous Avatar because he would have acted to stop his war plans , begins a genocidal assault on the Air Nomads . Sozin knew that the new Avatar would be an Air Nomad , and eliminated them in order to ensure his domination of the world would not be interrupted .
Simplified: After his disappearance , Fire Lord Sozin , in past friend of Avatar Roku , knowing that the new Avatar is an Air Nomad , launches a genocidal campaign against the Air Nomads in order to ensure his plots for world domination will not be interrupted by the Avatar .