There has never been an illness that kills 100% of the population. There hasn't even been a disease that kills 100% of the people that suffer from it, in a short enough time to prevent them from having children.
To kill off all its hosts in a short time is not beneficial to the disease organism; it will run out of hosts and die out itself.
It may not be so now, but there was a time when 100% of AIDS patients died; but they lived for years before even showing symptoms, allowing many of them to reproduce. Besides, it was not easily caught by casual contact.
Now maybe you are thinking of weaponized diseases. Smallpox might kill more than half of its victims; but those who survived were immune. Well, maybe somebody would engineer its genes to kill all patients - that might kill everybody, but I wouldn't count on it. Some would manage to avoid it, and some would survive it, and rebuild the population. It's not as easy as it sounds.
Then there is the fact that disease contagion is not easy to control. You can't limit the contagion to your enemies. It will get back to your own population.
The USSR is known to have had huge stockpiles of the smallpox virus, among other things. But they didn't dare use it.