Designing an electronic 'mosquito' among my arsenal of 'bugs'. Some work as remote ears, others as eyes. This mosquito is the assassin. He can fly into a room and when he 'bites' someone can inject a small amount of liquid. What substance could kill a human, from immediately to 2 days after the injection.

Bonus, what substances could just incapacitate a person for at least 20 minutes? 

This is Mosquito sized and as such the payload needs to reflect that. Is Ricin the most likely candidate? Can it even do this in such a small amount?

If 'spraying' the substance into eyes or mist for inhalation, that could work too!