The Horror, the Horror
There are a lot of different emotional approaches to writing transformational fiction. In some, a deep-seated wish or fantasy is fulfilled, while others deal with transformations that take place for purely pragmatic reasons. However, probably the most fertile transformational theme of all is TF as horror.
One need look no further than tales of lycanthropes and vampires in order to find horrific TFs. Vampires recruit normal humans, who become subject to unholy urges and are in essence cast out of normal human society. Werewolves become bestial killers when the moon is full, and must live in shameful horror the rest of the month at what they have become. The persistence of these mythical creatures in human folklore speaks eloquently of their symbolic resonance with the essence of the human condition, of the evil which we know lies within ourselves. Here, transformation is used as a powerful symbol, a sort of spiritual or metaphorical unmasking of what lies within. Some werewolf and vampire tales are among the most powerful tales ever written, regardless of genre.
Another commonly used motif in transformational horror is that of transformation-as-punishment. People are changed, either magically or through advanced technology, into a form appropriate for punishing certain misdeeds. Into this category fall the multitude of tales about rapists who are forced to become female, for example, or about murderers changed into ravening beasts. These tales too have deep roots in our folklore, and can be very powerful indeed.
Writing horror is quite unlike writing any other genre, in my limited experience. The ability to write horror well requires above all a sense of honesty; the work must have a sense of inner truth about it that transcends the limits of plot and setting and characterization. As the highly successful Stephen King has pointed out, good horror stories are virtually always morality plays in which good struggles, sometimes vainly, against evil. That 'good' does not always win is almost unique to this genre. Indeed, there are many characteristics of horror that are rarely (if ever) encountered elsewhere in literature, and the would-be horror writer would do well to keep some of them in mind. Horror is the study of the seamy side of the soul, the ugly underside of our consciousness. It is the place where we examine the ugliest of truths and the most unthinkable of thoughts. It should not be surprising, therefore, that with this unique 'mission statement', the author must employ several very specialized tools.
All SF/Fantasy requires a willing suspension of disbelief on the part of the reader, and in this regard horror is very much part of the family. It is incumbent upon the writer to make it possible for the reader to forget reality for a time and journey to the dark realm prepared for him. It is essential that the reader's journey be virtually seamless, that he cross the threshold from what is to what might be without even realizing just precisely where he left the path and began walking the forbidden ways. As always, your characters must be believable, your prose economical, and your settings well thought out. However, with horror something more is required. Because you are going to visit very dark places indeed, good horror requires a certain level of intensity that the authors of other sorts of works need not necessarily achieve. Because a horror tale as a rule is even more a drama played out upon the stage of the human soul than other types of fiction, you must have your reader very thoroughly hooked indeed. You need to make him identify wholly with your protagonist, you must provide enough action to hold your reader's attention, and all the while you must lead him further and further into the gloom, ever so gently lest he realize that slowly... oh, so slowly... you are turning out the lights...
Then, once he's there, you absolutely, positively need to slam him right in the middle of the forehead with some sort of emotional two-by-four.
It is no coincidence that a horror writer has license to write about things that no one else can write about, and go places that no other author is allowed to go. In my own modest attempts at writing horror, I've murdered several children, committed genocide and engaged in sexual congress with corpses. I've sunk sharp blades into eye sockets and described what it feels like when said blade scrapes around the bony wall of an eye socket while seeking entry into the sanctum of the brain. I've written of sinking a power drill-bit into someone's kneecap, and of the little spirals of bone that come gushing out with the blood. You can't turn back at the moment of truth, you can't pull away from the essence of what you are about to do. Flinch when you are staring down evil, and as a writer you are lost utterly and forever. Violate not one but all of the taboos; commit not one but all of the seven deadly sins. For as a horror writer, it is your special franchise and role to commit these crimes in your mind, to open these dark doors for careful examination by us all. Rape a priest; I have. Torture small animals. Tell loving lies to children, and then slit their throats. Fall in love with a demon, become something unholy, betray your mother's soul to an eternity of torment. Do all of these things, and above all do not flinch away when you are doing them! Tell the truth about evil in loving, lascivious detail, and in so doing bring a real shiver to your reader's soul.
After all, that's what he signed up for. It's what you owe him, when you undertake to write horror.
So, eat the forbidden fruit, and savor every morsel. Sink the knife all the way home, then wriggle the blade around a little just for the sheer joy of it, so that your victim squirms in agony as he dies. Trespass upon the unholiest of ground, kill that which is good and innocent and journey ever deeper into the darkness. Do all of these things, and your readers will thank you a thousand times over for what they have learned about themselves along the way. You will have done your full duty to both them and to yourself.
Of course, your fans may look at you a bit oddly from then on...