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.