As for "Don’t be afraid to hit hard. ", don't worry
@Jo Zebedee that will be the easiest part
. A bit harder will be to make the reader to love them before I throw them to piranha. Or monsters. Or elements of the nature. Or simply behead them by a bridge on a speeding train.
By hitting hard, I don't mean what you DO to them, but HOW you do it. It's easy to think horror is all about the what-happens when it's not. So, for instance, I have a few scenes of torture that have stuck in my readers' minds, apparently. But, really, they're not that hard hitting in terms of what happens to the character. I've read much, much worse happen to characters in books. The reason they're so hard hitting are multiple:
1. Closeness of point of view. We're in the victim's head for much of it, and in another witnesses' head for the rest of it, one with a deep conscience. There's no escape. Which means there's no need to hit hard. In fact, the worst scenes I took out cos it had gone quite far enough.
2. Empathy with the character. The reader knows him very well. They've followed his path through the book. We don't expect our heroes to be the ones destroyed - even in horror, we rarely stick with the victim-being-the-hero (I'm happy to be challenged on that one as I think it's a really interesting thing to explore - how often the hero is the real victim, not a pseudo-one to face their ordeal-to-make-them-a-hero) - so we have safe buy in that then gets ripped apart.
3. The readers' own mind. Much worse did happen to this character. I know it, and the reader knows it. They've been shown it in different hints - in dreams, in flashbacks, in intimate conversations where the worst is revealed but never shown. Here's the thing: there is nothing you can show a reader that is more frightening than their own imagination. For some reason, mutations and mutated people really trigger anxiety in me. All it takes is a hint of that happening and I'm out of there, and I have a pretty strong stomach for most thing.
4. Outcomes. Harebrain betaed this book for me, many years ago, and he said I had to nail the depiction of the character after the awful events - by showing, and showing well - in order for their impact to really work. He was right.
So, when I say hit them hard it's not about what you do. It might actually be more about what you don't do, and I think you might do. Or, indeed, how you tell us what you have done and what it does to the character.