how could they do it (not kill everyone)

1) rogue state releases a self moving self modifying AI into the wild. It is trained on how to hack, monitor new vulnerability updates, scan code bases to find new vulnerabilities. It constantly replicate and hides in systems so it will be extremely difficult to clear.

2) it hacks into public infrastructure taking down traffic, power, water, air traffic control, communications, etc.

3) all the things that preppers worry about in a lights out scenario from an EMP start to apply.

4) All the people on meds/machines start to die. The just in time food pipeline immediately empties out. Water stops flowing, sewage backs up.

Its hard to say how bad it will get because cars will still work so some transportation of food, water, fuel can happen. If it happens in the winter it would be much worse than in the summer.