In one of the reports Xehanort descripes that he did some experiments with his test subjects, and that resulted to the subjects heart to collapse:
What do you think these experiments were in nature? Did he somehow inject darkness into the patients veins, used so machinery, let the already born Heartless devour the still human patients? One way could also be that he used the Goat Keyblade to mess with the patients heart like he did withVentus, but since he seems to summon that when he attacks the other apprentices during DDD's flashback, maybe he did not have access to it yet back then?The experiments caused the test subjects hearts to collapse, including those of the most stalwart. How fragile our hearts are!