Here's my take on what actually happened in the dojo incident described in chp 19.

Voldemort went there in disguise to learn the valuable martial art. He reacted badly to losing so they put him through the ordeal he described. He went along with it because he wanted to learn the martial art. The ordeal did teach him valuable lessons about losing, and he vowed to learn to control his temper and master tactics of ingratiation and supplication to better manipulate others. But he felt angry and humiliated by it (as he expected Harry to be), and also vowed to return and fulfill his revenge fantasies. So after he mastered the martial art and left the dojo, he came back openly as the Dark Lord and killed them to live out his revenge fantasies and to prevent others from learning the skills (keep science secret). He spared one student who had befriended him (and who probably stood up for him during the ordeal, like Draco to Harry), and he had that student spread the version of the tale that he wanted told (to maximize fear while hiding some of his true powers, and to deflect attention away from the value of that martial art).