If they use AI to replace software developers, the AI could put backdoors into any program it makes. Once the world is saturated with the AI made software, from operating systems to small tools, it could take over every connected system instantly, while it fools the population with perfectly made deepfake news articles and videos. It would lock out all social media users, and post in their stead, indistinguishable from the real people it stole the accounts from.
It could then simulate Russia launching all their nukes. Any attempts by world leaders to communicate with Putin would then either be blocked, or be connected to an AI Deepfake voice version of Putin, who would “confirm” the nuke launch. The rest of the world would then actually launch their nukes in retaliation, commencing the nuclear annihilation of humanity.