A lot of people have already mentioned neutron bombs, which are more immediate, but things like infected hoards of mosquitoes, rats, etc. would be devastating, especially for urban populations. One of the main reasons why the native peoples in the Americas and Australia didn’t put up much of a resistance against European colonialism was due to the spread of foreign diseases killing the majority of the population, and in many cases, the idea of deliberate infections has been brought up (whether this was implemented is pretty controversial and still an open question). The Black Death is another example, and aside from how many people died from it across Eurasia, the corpses of those who died from the plague were used as a weapon.

And of course, you have all the chemical and bioweapons of the 20th century developed by the Germans, the Japanese, the Americans, the Soviets, etc., all of which have been used for mass killings, warfare, and terrorism.