I'm pretty sure evil governments already know this. If nothing else, it's been the plot of many popular stories for decades.

The main deterrent here is that it's too easy for the leaders of the evil government to end up being infected too. Even if you target a virus at specific genetics or make a secret vaccine, it's impossible to predict how it might evolve once it's out in the wild. At least given current technology, viruses can't be controlled well enough to make good weapons.

They were most probably aware of that yet