One theory I've run in to in psychology is the idea that we don't want to believe monsters are human beings, because it forces us to acknowledge our capability of monstrous behavior.
Humans have very selective empathy. We only innately have empathy for those we either know on a personal level, or whom we see to be inside the "tribe" so to speak. As civilization has progressed the circle of beings that we consider worthy of consideration has steadily widened, but our ability to dehumanize the "other" is always present if backsliding does occur.