The problem with all weapons of war:

is that once something is invented as a weapon of war (no matter how horrific) it tends to be used on people somewhere. The same is likely true of nuclear weapons (Hiroshima and Nagasaki) and slaughterbots.

The concept is using AI facial recognition combined with cell phone GPS to kill specific people on earth with a shaped charge or just a close range bullet. AT present I see no reason why this could not be done with existing technology on earth right now.

Here is my question for you? Are you sure this isn't already happening in 3rd world nations?

The problem is a little AI facial recognition drone could simulate a bullet to the head. If no one saw the drone at night how would anyone know? Drones often have infrared (see in the dark ability) so how would anyone know? This could be something as small as 5 inches across in the size of the drone and could fly through any opening big enough for it to enter even when a door was opened or closed if it was fast enough.

Here's the problem: Even with existing technology (without inventing anything else) this could be done.

How you stop something like this in a mass attack?

First you have to know where the attack is going to happen and then you need an Electro magnetic pulse which would wipe out everything electrical for miles around that area.

Either this or jamming the signals on the drones somehow. But, this wouldn't work if there was resident AI on board the drones. However, if GPS signals from satellites could be interrupted or jammed it couldn't navigate to it's intended targets.

So, in this sense there is a defense against this sort of thing but only if you know where it is going to hit ahead of time.

Later: After thinking about this a little more I realized you might be able to design AI to use landmarks instead of GPS for navigation thereby eliminating the need for GPS. If someone did this then cutting off GPS would have no meaning at all to the drone because it would be using landmarks instead of GPS for navigation. However, it still would be vulnerable to Electromagnetic pulse. If a pulse large enough occurred all drones and passenger planes would simply drop out of the sky if they were hit by a big enough pulse and everything inside of the pulse area that was electrical would simply never work again. The pulse fries basically everything electrical out of service permanently if it is on at the time.