A Red Goo Singularity would require a vast increase in drone precision, numbers and simplicity of piloting/managing.
These plans are in the works:
"Human control over a swarm can take many forms. Human commanders might develop a detailed plan and then put a swarm into action, allowing it to adapt to changing circumstances on the ground. Alternatively, human commanders might establish only higher-level tasks, such as “find enemy targets,” and allow the swarm to determine the optimal solution through centralized or decentralized coordination. Or human controllers might simply change swarm goals or agent preferences to induce certain behaviors. If the cognitive load of controlling a swarm exceeds that of one person, human tasks could be split up by breaking a swarm into smaller elements or by dividing tasks based on function. For example, one human controller could monitor the health of vehicles, with another setting high-level goals and yet another approving specific high-risk actions, like use of force. " - Source
It would require at least one nation-state, or several nation-states, to develop vast drone armies on the level of their nuclear weapons arsenals: weapons so terrible that could destroy the world many times over.
Based on strategies developed by modern military theorists, the best use case for drone total warfare would be to quietly pre-position them inside of enemy territory and then launch them all at once to take out critical infrastructure and people.
A writer like Harlan Ellison (creator of 'The Terminator' idea, at least according to his excellent lawyers) could imagine the terrible-ness of small swarms of explosive suicide drones targeting people's foreheads, of drones that sneak through open doorways and subways and then release poison gas once they've found a sufficiently dense crowd of people.
This kind of pre-placement would require drones that could maintain and power themselves, perhaps feeding off of the host country's power grid or going into some sort of sleep mode.
It would also require some form of artificial intelligence, to either take over the entire operation or to enhance a human being's ability to manage the complexity of pre-positioned drone swarms ensconced in the fog of war.