Writing about the zombie apocalypse, I create an entire world filled with ravenous undead that also serve (at least in my world) something outside their own desire for human flesh. There are other monsters out there that you haven't seen yet, new horrors to test my characters. The whole story is in my head, and I have to create human characters who are just as much monsters as the zombies waiting outside the wire.
"Why do you write about this kind of thing if it does this to you?" Good question. It's a natural thing to ask. If what I write doesn't scare the shit out of me sometimes, or at least make my pulse race a little, how the hell do I expect it to do the same for the reader? It isn't a matter of writing what I know, it's a matter of writing what affects me.
The truth is, zombies scare the crap out of me. I can't binge watch TWD. Add to that a dose of respect for the mythos of HP Lovecraft, and you can imagine the horrors my brain can create. It isn't so much that I watch TWD and go "Oh, that's so horrible!"
No, I watch The Walking Dead and ask myself "What if...?"