We hated each other so much, that despite international agreements to the contrary, all of the "great" nations poured millions into biological warfare.
The dream of those demented scientists was a weapon which would kill off their enemies but not hurt their friends; a virus that was designed to skip one nationality, while killing all others.
One particularly "pure-blooded" nation realized that dream. They created a perfect killing machine; airborne, long-lived and highly contagious, yet selective enough to be considered a genuine racist. It spread fast but killed slow; allowing its victims one last day to clean up their affairs, shut down their societies, and make their peace.
More than a few chose to use that last day for vengeance.
5.9 Billion died from that plague. The rest died more brutally with plague-ridden hands around their throats. No one survived to greet the next dawn.
Yet our legacy, the virus, lives on.