Think about it he was always antagonizing Roxas and Sora, Xehanort is the main antagonist of Kingdom Hearts, so what if Seifer was feeding Xehanort with his negative emotions, thus causing Xehanort to find Seifer a worthy vessel. Thus causing Seifer to be the 13 Seeker of Darkness.
Hire me as a writer Square Enix
Hire me as a writer Square Enix