Angels were all originally part of the same race before the war in heaven, when the ones that rebelled were cast out and sent to hell. The humans on earth were similar to angels, but without the essence that made them divine. Among the angels that remained in heaven, some took a fondness for mortals and bred with them to produce mortal offspring called the nephilim. These creatures were monstrous giants that were bestial in nature and appearance, committing evil acts like cannibalism and murder. They were eventually wiped out, but the lesson learnt was that divine and mortal essences cannot mix.
However, this appears to not be the case. Devils, angels that were cast down from heaven, are still able to breed with mortals, producing cambions. The resulting offspring is still a monster, committing evil for its own sake. However, their bestial nature is internal rather than external, as they look completely human. Legends speak of the anti-christ, a being who would manipulate world governments and turn mankind to evil acts that have yet to be seen. This being would be the son of Lucifer, once the greatest of angels and responsible for the war in heaven. He now ruled hell, as he was cast into it with the angels that rebelled with him.
How is it that devils spawn monsters that look human, while the good angels produce monsters that look like monsters?