You wouldn’t want to release a full-scale deadly pathogen, because then you would endanger your own population as well as your adversary.
But instead, you release a pathogen (like Coronavirus) that is just deadly and contagious enough to cause a worldwide panic, shutting down countries and economies and threatening to overwhelm hospitals and emergency responders.
Then you would watch patiently as vaccines are developed that everyone would need to get to resume their lives and jobs. Of course, the vaccines get rushed through development, approval, and distribution in order to calm the fearful and disenfranchised populace.
And here is where you would have your adversaries by their you know what, because if your agents can then infiltrate the vulnerabilities of the overwhelmed and unprepared, civilian pharmaceutical companies, their supply chains, or distribution networks during the emergency, and contaminate the vaccines with a really lethal pathogen, you would ensure that all your adversaries are eliminated (from their leadership on down to the blue collar workers in society), while your own population (which wouldn’t get those vaccines from abroad) would be absolutely fine.
Maybe even the first dose is intentionally passed through okay to lull the population and its leadership into the trap, but then the follow-on doses are tainted once the flies are already in the spider’s web.
Your adversaries are now sick and/or dead, but their countries and infrastructure are fully functioning and in place for you to take as spoils.
World domination, it can happen as plain and simple as this.
Look around you at what’s happening in our times. Mad men or genius, but this is more than any science fiction could offer. 😉