To get a sim eating plant click on one of your garden plot thingys where you grow vegetables. Select any vegetable with the green monster symbol (looks like a crocodile's jaws).
Get multiple sims to plant it because it won't always make a sim eating plant.
That's all I know, I've never got past that part, but that's how to get a sim eating plant, though I don't know what happens once it's grown. (It was only today when I planted my first sim eating plant and it isnt grown yet...). All I know is that it WILL NOT kill or remove your sim, though it will probably give you a penalty like not getting the simoleons or having to buy the plant an item.
Hope this helped