Becoming a monster plays into an aspect of horror that I'm very fond of: body horror. Becoming a monster sometimes involves physical changes - shifting, or things under the skin. One of my favorite incarnations is in Homestuck and Homestuck fandom, where there exist Helmsmen: high-powered psychics capable of directing a spaceship with their minds. Mostly they are chained to the ships, stripped of free will, and wired directly into biotechnological interfaces. Fanworks are frequently somewhat horrifying.
Body horror taken to this kind of extreme is its own kind of externalization - of different things for different people. Puberty is generally horrifying, with all of the growing and hormones. For trans* people, who sometimes experience their changing bodies as deeply and irreconcilably wrong, it can be deeply horrifying and an utmost betrayal. Disability, severe injury, and assault can all be traumatic. They can all be body horror, and expressing body horror as a plot device, as a way a character becomes stronger can be ways to explore the more mundane sort of body horror at enough remove that it's just fascinating. Body horror can also be a way to explore the ways we take - and deal with - damage.
I haven't generally been fond of the product of writing as therapy, but that has somewhat changed - body horror in particular has let me read stuff that I can relate to - and I've also come to a slightly different appreciation of the ways we use our experiences to create art, which is well-articulated in Amanda Palmer's review of The Ocean At The End Of The Lane. I've found that I really like when things are fine-ground and spit out as art that's appreciable in its own right and not just as a reflection of the creator.