All of those experiments, as cruel as they are, had the underlying motivation of being performed for knowledge and power. Kefka, on the other hand, had no motivation for his poisoning of Doma other than pure sadism. Hojo is cruel, but Kefka is crueller because the only thing he cares about is causing other people suffering. Even his pursuit of power is only carried out as a means to the end of allowing him to cause people even more suffering.