Autonomous drones have killed human soldiers for the first time

Right, I agree it is fuzzy. I just think, from an ethical standpoint, it is better to think of them as mines that have more mobility. Reasoning from the other end as projectiles which are slower or have more guidance seems to invite too much optimistic thinking about the level of control. That the victims will be as intended rather than quite indiscriminate and unpredictable.

I realize there is a full, multidimensional continuum here.

On one end are directly-aimed weapons that do their damage while still being aimed by the operator. Their risks include collateral damage limited to things like aiming errors, effect radius, or continuing down-range beyond the target.

Further out are messy things with more active guidance that can turn and seek the target and potentially go off course. But their time to target is still quite limited and more or less being observed by the one who fired it. The risk expands with its potential "cone of maneuvering" and travel range.

Then you get into these things with long dwell times and autonomy where the eventual targeting event happens without supervision and is greatly affected by things happening in the environment which the operator cannot have really predicted nor controlled for. The longer time in operation increases the risk not only from wandering/guidance but from how much the environment can change before it performs its final targeting event.

Another example in this category could be chemical and biological weapons. There is a lot more uncertainty in the targeting effects due to the way it disperses in the environment.

The difference is in "active"/intelligent versus "passive"/dumb targetting that's performed by the machine.

The missile, once fired, has the general vicinity (if not the exact position) of the target and is armed by the operator. Therefore, the operator is fully accountable for the targetting. Same goes for the landmines, once placed. Hitting civilians is reckless at best, and negligent at worst.

An autonomous weapon system (AWS) usually means that the system, once deployed, can do the targetting itself over any arbitrarily bounded location. An AWS can continue finding targets as long as its hardware allows it. For kamikaze drones, it's one time; for other drones, the ammunition & battery are the limits.

We currently rely on human targetting because we assume that A) humans are able to distinguish between legitimate and illegitimate targets "well-enough", and if not, B) at least we can hold them accountable (e.g., punish them for war crimes).

An AWS provides a layer of plausible deniability: the operator can claim that the system wasn't developed well enough, while the developer can claim that it wasn't used as intended. Given the inscrutability of modern computational intelligence - i.e., visual-action neural networks - this could potentially lead to very worrying incidents.

From a technical POV, the difference between a manually operated drone and an AWS drone may not be massive. From a military POV, it's just another legal lethal tool in the arsenal.

But from a social/civilian POV, the use of AWS is still 'not normal' and opens a can of worms. Targetting while evading counterattacks and crimes successfully is a bottleneck for manual operation. That's no longer the case with AWS: build 20 thousand drones, for example, and you can trivially win by overwhelming any manual defense of frontlines or cities. And knowing the history of human warfare, winning can range from relatively bloodless regime changes to utter destruction of the loser's civilization.

So, the best outcome is similar to nuclear deterrence or MAD: as long as everyone has 20 thousand AWS drones, they're safe.

This is a much more difficult distinction to make than you're letting on. Cruise missiles offer no quarter, but manually operated drones might (though there is often no way to capture the opponents). The question is what is the difference between the two weapons systems...