Well there are many ways to kill someone but the first that came to my mind is using "Tetrodotoxin" (found in the blue-ringed octopus and the puffer fish). It is capable of killing more than 20 humans in less than a minute. If you were to walk by someone and simply inject them with a small needle they would be paralyzed and dead in minutes. You could also slip it into a food of some sort.
