A protagonist (who we’ll call Seleriki) and her team manages to kill the leader/a significant member of an ancient evil group. The evil group’s goal is to wipe out humanity because of “how sinful humans are and how they interrupt some natural order”.
They (more specifically Seleriki who did the execution) used a deadly virus. However, the virus sample spilled (either secretly or obviously) and the heroes end up starting a deadly plague.
tldr: heroes accidentally release a deadly virus while killing the leader of an evil group
I mean, it’s okay, but it sounds like you’re unloading a totally different story. A plague is a major event, and readers might be disappointed if it goes away so soon.
The fact that the heroes are weak, and have to resort to “underhanded” methods might dissapoint readers, unless the story has more examples of the group making up for their weakness with other like wise “underhanded” methods.
If the story is about a group of heroes, that actually have worse morals than the average human, and also resolve to underhanded methods, then I like this.