Cheese is not ordinarily thought of as a deadly weapon. Also, it is true that "poison" could be drawn in a picture in the form of a bottle with a skull and crossbones on it. But given that a very common method of killing rats and similar pests is the use of poisoned bait, and cheese is a popular form of bait for rodents, the wedge of cheese in Bang's word balloon listing methods of killing could be taken as standing for poison as a means of killing.