What about poison?
A machine capable of carrying a toxin to a pre-determined target via GPS could be a winner. Depending on the target the toxin could be a gas, liquid or powder. A free dispersal mechanism (the downdraft from the rotor blades on a chopper) is a handy benefit.
I guess you'd have to pick your toxin pretty carefully though to ensure adequate effectiveness.
I would have to suggest though that if mass deaths rather than targeted assassination was the aim then disrupting a rail network and releasing gas onto crowded stations or in carriages would be far more effective - eg. the Japanese cult Aum Shinrikyo which used Sarin gas in just such attacks.
Biting the hand that feeds IT
