While your robots do sound like they could kill a lot of people, I feel like this is a bit overblown. For a start, for "apocalypse", they would have to be produced in massive numbers, i.e. by a nation state-level actor. State-level actors can already make nukes or bioweapons.
What's so hard about manufacturing a few million drones? With a relatively modest amount of money(relative to, say, developing a nuclear program or chemical weapons program), you could hire a contract manufacturer to spin out a few million drones. They probably wouldn't even ask what the blade/needle was for, although it would be easy to craft a cover story(i.e. "They're for mass inoculation/slaughter of livestock"). Update the SW and congrats, you now have a weapon of mass destruction. You can target people by face, race, location, etc.
There's a reason the US military has been playing with drone clouds for years.
It's not hard for a nation to manufacture or order millions of drones. It's also not hard to obtain enough rounds of ammunition to shoot millions of people. It was decades if not a century ago when the primary obstacle to a nation killing millions of people ceased to be "inherent technical difficulty."
It's not hard to shoot millions of people, but it is hard to raise the army required to do so. It is hard to do so without raising suspicions. It is hard to do so without damaging infrastructure.
I rogue state, or even a non-state actor, can leverage cheap drones(assuming, of course, a solution to the power problem) to obtain a weapon of mass destruction. Drones are fundamentally different from bullets.
