I presume The Walking Dead is on-topic because the zombies are fantastical. Or if they weren't fantastical, the result of the zombie virus would be considered sci-fi. Fine.
So do we need to apply one of those criteria to all horror works for them to be on-topic?
For example, is a horror film with a realistic serial killer off-topic? Or just a deranged individual? (Like perhaps Saw? -- I haven't actually seen it, so I may be wrong here)
Or is horror automatically on-topic regardless of plot or elements?