As in any field, you can't make a sale unless you have the right market, and in horror the variety of fiction has made understanding the markets something of an arcane art. As a neophyte you might feel like you need a guide to the vocabulary of dark fiction. There are arbitrary, debatable labels that folks use for different categories of horror. Differences will occur in definitions, but I hope this helps to orient you to the dark. A market looking for extreme is probably far beyond what you can imagine if you haven't been exposed to it.
Some extreme is written just for the joy of grossing people out.
Splatterpunk , by the way, doesn't exist anymore, if it ever did. Obviously psychos on rampages fall into this category, but it is just as often more subtle. Since the reader's perception is sometimes altered by exposure to an insane viewpoint, psychological horror can also deal with ambiguous reality and seem to be supernatural. It often leaves the reader wondering what is real and what isn't. Here you'll find ghosts, demons, vampires, werewolves, the occult etc.