A drone with a gun is unrealistic, I think. The stability of a light, air-borne vehicle is just not good enough. A drone with fragmentation grenades, on the other hand, could work well and pretty much erase all evidence.
I'm mostly commenting because I think the "small drone with a gun" idea is interesting; no idea how it would play out in practice.
If you had a drone with a weapon, I would think you'd want to fire on someone using some variation of Tracking Point style hardware [1], except partially self aimed. Basically see the target, paint the target, drone uses internal sensors to detect a brief instant of random stability and fires the weapon
On the show Sons of guns, they already made a drone that can shoot a pistol. Not very hard to do. Drones can withstand very high winds and hold position very easily, lots of hand guns have very little recoil and can easily be shot from a drone with very good accuracy.
Fragmentation grenades are presumably fairly difficult to come by. I think it's laughable to worry about an autonomous or remotely piloted drone assassinating people with fragmentation grenades when a normal firearm operated by a human is extremely practical and seems to work well enough. The latter is designed for nearly that exact purpose, and refined over centuries. They're cheap, easy to use, easy to get (in some places), and reliable.
when a normal firearm operated by a human is extremely practical
Again, doing a crime and getting away with a crime are two enormously different things. The truth is that the majority of humanity can commit terrible crimes with utter ease. There is absolutely no question about that.
Most of us don't, however, because of morality and our nature. The rest don't because they don't want to spend the rest of their life in jail, and for major crimes the probability of getting caught is enormously high. Things that reduce that probability (and a device that means you don't have to be on or even near the site, and can extract 100% of evidence with a very high degree of success, does that) open up opportunities for people to do such crimes.
As a parallel, someone mentioned bitcoins, and bitcoins absolutely opened up the world of casual extortion, because suddenly you don't have to go to a mailbox or meet in a park.
