For sake of the discussion, let’s set the boundary at something that can reliably kill a person, say, 9mm or so.
Is there anyone out there hunting coyotes or pigs with a UAV?
Well, it’s not exactly a Predator, but there’s this old chestnut—a (large-ish) remote controlled helicopter armed with an AA-12 automatic shotgun.
Aha! It looks like they even make an updated, commercial version.
I would be amazed if someone, somewhere, had not done this.
Oh yes… this guy has.
Fake.
There’s the Roman candle drone, which fits a certain definition of “armed;” it may not kill you, but it’ll burn you pretty good.
Hunting animals from the air is illegal in most states.
Big time NO NO.
If they did, I doubt they would tell anyone. Especially if it had machine gun capabilities and that would be needed IMO. Semi-auto would work better with an gunner doing hand held and is much cheaper and easier to hide and not sitting there displaying the NO NO you have committed and if you can afford a helicopter you probably also have lawyers on full time to tell you what a bad idea it is.
YMMV 
By the same token, Israel Aerospace, General Atomics, Boeing, and Lockheed are those of private citizens (as are most of these 391 companies). You can better ones and cheaper too.
Nice 2013 Congressional Research Service PDF on the industry.
It seems like OP meant “private citizens who use them” in certain circumstances, and if they can get away with it.
I seem to recall that there are already laws that prevent civilians from mounting functioning guns on historical aircraft (like P51 Mustangs). Wouldn’t these apply to drones?
Found this tactical UAV industry guidejust now. Hovercraft–the more likely platform now for do-it-yourselfeds–is another page there.
Just so you know.
Before Red Jacket firearms went off the air they had a quad with a remotely fired 9mm pistol on board. But it only fired straight down.