L.Dutch has made an important point about the people living in far flung, uncivilized corners of the world. Unless you use a global airborne poison, these folks will not be affected. But a global airborne poison would be immediately detected and your agents will be exposed.
So, we have either of two options:
Genetically Engineered Pathogen
First you collect all the viruses and bacteria which only target humans (smallpox, typhoid, whooping cough etc). Now you enhance their capabilities and make them more deadly. For example, you could create a flu virus which makes infected cells produce and secrete ricin or abrin or shigatoxin into the bloodstream. This would make flu a killer. For the worst hit, you could work on the HIV virus and enhance its functioning so that it doesn't have to be injected into the bloodstream to function and would start destroying the immune system when inhaled. The possibilities are endless.
Once you create your super pathogen, you release it in the atmosphere through aerosols. This would target all the human population and would make it extinct within a few generations.
Here again, you have endless possibilities. You could go the painful route, by spreading carcinogens into the environment and create a gigantic spike in cancer rates. This would quickly overwhelm all national health budgets and situation would soon be out of control. You would easily kill 80% of the population within a few decades or so.
Or if you want to go the classical poisoner's way, you could use a slow poison such as copper sulfate, sodium selenate, corrosive sublimate etc, grind them very finely and spread them in the atmosphere through aerosols.
The downside of this method is that these mineral toxins affect all complex life alike, so all large animals in the ecosystem would be getting affected alike. You would have a lot of dead animals to dispose off, at the end of the day.