I'm not quite sure where the theme comes into this, but as a simple wave defense with a floating murder sword it's an interesting idea.
The hitboxes a little vague on the sword and the enemies/humans. Which makes it a little difficult to avoid hitting the humans, Which to be fair I think was part of the point, but I think it would be better to have the hitboxes very precise and just rely on having enough humans vs monsters close together to make it difficult.
It's also an interesting way of handling a health system, though I feel like getting double punished by losing health when you kill a human was a little annoying. considering your'e likely already going to lose a life or more, if you don't kill that human, it removes a bit of strategy and makes it a bit more of a precision game. With some really vague hitboxes making that quite difficult.
Overall though an interesting endless wave game base. That adding some variety to would definitely help spice up.