I'd like to think that her teacher knows that, and has done similar things to his other students as well. It was just easier for him to arrange Kaliyah's death since she had a few enemies. For the ones who died then he writes them off as failures, since they would lack the immortality of the Dark Sign to make best use of his knowledge. Everyone else he had killed who turned into an undead he would have trained, inadvertently or otherwise, so that their skills won't be lost just because they died.