An exterminator is someone whose work is to kill a whole group of living things so they are gone. In ordinary American English, if a neighbour says “I called the exterminator,” they almost always mean the pest-control person — the one who comes for ants, rats, or termites. That is how people really talk, even though the root is the same dark verb. When the word is about people, it is rare and very harsh: a person who carries out mass killing. Though actually, there is almost no everyday middle sense: it is either the bug job, said without drama, or a history-horror word. Students should hear the room before they pick it.