You seem to be only considering the moment of crime and disregarding what happens before and after it. A tool that almost completely removes risk of getting caught and need for alibi will surely be seen as more practical by all except mad and ridiculously dumb people, regardless of how less effective it is.
A drone leaves all that bomb trace leaves and more - its construction, hair skin and dna traces of the builder, even its radio control details. So its not perfect; its more like leaving the rifle at the scene.
And the idea that shooting a gun brings down some hailstorm of official response is myth. Fire a gun in most places and ... nothing will happen. Folks will rush to the scene of any murder, but will only find the sniper's location after hours or days of investigation. It won't be like the NCIS show where the investigator glances around and says 'must have come from that rooftop; its the only secure sniper post in a half-mile!' You can shoot from nearly anywhere with line-of-sight, from the partly-open door of a parked delivery van, without much risk of discovery.
I considered the likelihood of getting caught as well. I mentioned that explicitly in the rifle example. It's fairly obvious for the telescope example. For arson, again, I don't know much about how arson tends to be committed, but it seems that a drone is not inherently harder to track than a car, bicycle, or person on foot, given the range, speed, reliability, and conspicuousness of those modes of transportation.
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.
Fingerprints, hair or clothing samples, asking nearby drone hobbyists if they might know anyone into drones who might want to murder the victim, sales records of retailers of drones or drone supplies, and of course the normal stuff, like who would want the victim killed in the first place.
I'm not a detective, but I suspect having the large, complex piece of electronic equipment that was used as the weapon would be a godsend compared to the evidence available in a normal murder or assassination investigation.
Obviously the plan wouldn't be "drop the drone at the scene of the crime". Why would anyone ever do that? I mean, of course they would minimize the reveal were that to happen (though note that drones are becoming absolutely pedestrian, so this notion that you talk to the niche club and find the drone enthusiast is absurd), but the rational plan would be that the drone would escape as well. Because its odds of escape, currently, barring mechanical defect, would be close to 100%. No one is going to catch a drone.
Perhaps. But a wlan card taken from a discarded/stolen/recyled/used corporate machine, then transported half-around the globe and sold for cash, isn't necessarily easy to trace back to a perpetrator.
