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And thinking about it, it's likely impossible to create a virus or disease that could kill large swaths of people. If it is supposed to be lethal, it has to spread fast and aggressively, which would reduce the incubation time. Reducing the incubation time reduces the rate of spread, improving the ability to prevent the spread, and also showing symptoms earlier. If it's airborne, it automatically loses a lot of its potential power. Every serious/deadly disease is transmitted physically via fluids, contact, etc.

In order to circumvent that, people are needed who thoroughly work on creating a virus that spreads fast, has a long incubation time and has a high lethality. For example the symptoms would have to be subtle enough, and be lethal over the span of weeks or months... like something that slowly destroys the heart, the lungs, the liver, the kidneys, the brain, etc.

Or: Which was also done in sci-fi movies, sterilization. It takes months, years, decades to figure out that someone got sterile.