Ygthkb

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 similiar to angels, but without the essense 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 humam. 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?

Hell is a pretty toxic environment. Being there over a long period of time has mutated the demons, causing them to look different from angels and passing on changes to their offspring.

First we are to assume that Angels and Devils can be classifiable by species.

Second that they are the same taxonomic species.

Third that humans are of a different species?

Clearly then the answer is that it is something external to their taxonomic definition that allows interbreeding, as (going by our definition of species) neither could successfully breed with humans.

If it is an internal, integral and automatic capacity, neither Angels nor Devils can be classified as part of a species at all because they can breed with things other than themselves, so to speak.

In summary: However it occurs, to define Angels and Devils by specie must necessarily be flawed.

It follows anyway, that whatever agent it is that makes their breeding with humans possible is altered either as integral to their displacement(from Heaven or to Hell) or as a separate function imparted to their nature, rather than that of the agent. In real world terms it's theoretically feasible to create an egg or sperm that 'pulls apart' it's counterpart and reconstitutes it in a form viable for reproduction making ad hoc inter-species breeding possible.

If we consider that what makes Angels Angels must be 'impartable' only from the Divine, otherwise the Devil would simply make himself and his Devils undevilly, it would perhaps stand to reason that an Angel cannot simply make more Angels via mating, as it is not up to them to make more divinities, but the Divine. It might follow then that it is relatively easy to make more evil, but not so easy to make more good, so to speak. Heaven has an immigration policy to uphold after all, and free passes would make a mockery of that.

Angels still bear the blessing of good, while human and demons not.

When angels and humans breed, the interaction of a non blessed with a blessed cell messes up the appearance of the result being, while the pairs blessed-blessed or non blessed-non blessed give no issues under this point of view.

First a biblical answer and then a fictional suggestion. Use which one you like!

I think the consensus of biblical scholars is that the passage about nephilim is super weird and hard to be sure about. But it seems pretty clear that it's referring to fallen angels doing stuff. As for the antichrist, in other verses (Matt 24:24, stuff in 1st and 2nd John, etc) basically any spirit that denies Christ is the anti-christ. There are theories that the antichrist mentioned in Revelation was actually Nero, who seriously persecuted Christians. See for Christians, we've been living in the end times since Jesus rose.

But yeah! For a fictional suggestion... It's a really big thing that Satan is a liar. It's so much a part of what he is that Jesus said it's his native language. So the demons' monsters are monsters that are lying about being monsters. The other ones are honest about it, at least. But it's gonna be hard to explain why an angel following God would want to make a monster lol. Hope that helps!