Man dies in an atmosphere of carbon monoxide because

It combines with the haemoglobin of the blood making it incapable of absorbing oxygen

Every poison has a particular trait that causes it to be poisonous. In the case of carbon monoxide, the trait has to do with hemoglobin in the blood. Carbon monoxide is deadly because it readily binds with the iron atoms within our hemoglobin. The affinity between CO and hemoglobin is 200 times stronger than the affinity between hemoglobin and oxygen. The preferential binding of carbon monoxide to iron severely limits the oxygen to bind to hemoglobin. As a result, less oxygen is carried through our system and we die.