"Only one individual capable of creating a super-pandemic": by hypothesis, this individual is morally good in some way. (Otherwise they wouldn't be having this internal debate about whether or not to release the information about how to produce the virus: they'd just release the virus!)

The ideal scenario is that throughout the rest of time, no entity ever manufactures the virus; the second-best scenario is that an entity does manufacture the virus, but only after there is some hypothetical defence against it. You are shattering forever the dream of achieving the ideal scenario (again by hypothesis, the virus is easy to make, and history demonstrates that if something is easy and destructive, eventually someone will do it); you're pinning your hopes on the second-best scenario. You are therefore implicitly assuming that "we get better at arming ourselves against the virus" happens faster than "enemies create the virus", which is by no means a given and must be weighed on a case-by-case basis.

By the way, this is the basic reason why I'm so sad about the existence of fully 3D-printed guns. There is essentially no defence against guns. The creators of the 3D-printed gun chose to spread the blueprints far and wide, to accelerate their advent. It's sad that human nature is such that this was predictable and inevitable.