There's no evidence that a doomsday virus can be created. There's a tradeoff between how infectious and how deadly a disease is. You would have to develop a two-stage virus, and that would require a large lab or labs with material being shipped there. Something humans would definitely notice. Even then, how would the virus reach literally everyone? People will take measures to not become infected once the threat has been identified, and some people and groups are isolated as it is. And no virus kills everyone. Rabies comes closest at 99%. But unlike what the zombie genre would have you believe, it has a poor means of delivery.

This isn't like vibe coding. The AI would have to make the virus in laboratory conditions and then disseminate it globally without prior detection.