Not an expert, but this has more or less already been done [1] and led to a temporary moratorium on this research.
To your last question, releasing such a thing would basically assure your (and your people's) own destruction in addition to your target's, as we've seen by how difficult it has been to control COVID. So you'd have to want everyone dead, and generally speaking the people who would be willing to actually do something like that don't have the resources and knowledge to make it happen.
If given the right resources, would it be feasible to synthesize a pandemic-inducing virus in a lab setting?
If so, a general question: why haven’t the villains of the world done so already?