The method of how people are injured and killed should not make a difference. But it does! It has to!
There is an agreement in place that tries to ensure that governments will only be able to take overt action - men, tanks, guns, planes - things that can be seen and countered. It is supposed to limit the damage that can be done, literally and politically. Partly because it costs a lot of money to do, and partly because other countries can say clearly "We see you, you did this" - which in turn is supposed to limit the lengths governments will go to.
Chemical warfare is banned so that no government can kill thousands of people without warning, invisibly, repeatedly, in relatively cheap terror attacks.
If you are happy to live in a world where governments can develop the nastiest of chemical and biological weapons without international condemnation (including our own) then you have utterly missed the point about the almost sci fi / horror movie ramifications.