At the moment, I don't have much here. I'll put something better later.
To get sims that behave like Zombies you need to make them angry. First, let em eat beans! No cooking allowed in the zombie house hold. Beds? Nope, sleep on the floor. Chairs? Give em a couch. Just one. Showers are optional, but hey just think. Stinky zombies! If an object editor comes out we can create things for zombies to play with. Like a corpse! It could act like a fridge, and the entrails could be food! Also, don't give your zombies any personality points. Or maybe a few in active and playful.
I found that zombies in your neighborhood can really upset your other sims. They come over to your other homes, and start slapping people. They start off with that friendly warm handshake, then beat the sims to death!
I locked all my zombies in a large room with a fridge and a TV and let them just filth the place up. Even with a neatness of 0 they tend to clean up too much .