Viruses mutate because they are constantly exposed to the immune systems of people who were already previously infected, so they have to mutate in order to dodg

Viruses mutate because they are constantly exposed to the immune systems of people who were already previously infected, so they have to mutate in order to dodge prior natural immunity. That isn't a concern if most people who get infected die. Moreover, the first SARS-CoV-2 variant took a long time to start mutating and even then it was only minor mutations. If you release an extremely infectious very deadly virus and you have a vaccine for it, then at that moment it's a Deus Ex scenario and whoever can manufacture the vaccine can take over the world because your enemies are now collapsing and will do anything to obtain the vaccine including giving up their own autonomy. Guess that's the theory, at least. As to why SARS-CoV-2: because it's airborne and can spread long distances via gas-like clouds (which is why lockdowns, masks, wiping surfaces etc didn't work). You can't really get better if you want to very rapidly infect a whole country.