Keep in mind that the activities of him and his colleagues during the war have yet to be disclosed by the British government, as they are still classified.
Kai was never wronged either. He only felt betrayed by Oogway because Oogway turned around to good, stopping Kai from taking Chi.
While Shen and Tai Lung held darkness, being told by their loved ones that their fate was to fail no matter how good they were then, set them off.
