The Chernobyl Nuclear Power Plant Disaster, which happened in the Ukrainian town of Pripyat in 1986, killed 31 people directly. After the disaster about 985,000 deaths, and 2,500 otherwise wanted pregnancies terminated due to fear of the mother of radiation risk can be attributed to the radiation leaked during the Chernobyl Disaster. Also 4,000 deaths related to cancer caused by the radiation leaked because of this disaster are predicted to happen in the future. So that's about 991,531 deaths in total.
Also Chernobyl spread radiation a lot further than Little Boy and Fat Man and the radiation significantly effected land as far as Italy, Moldova, Switzerland, Norway and Germany, and also brought some radioactive rains to the UK and other countries.
Though much worse things could've potentially happened/could potentially happen:
- The Cuban Missile Crisis starting a Nuclear War
- If the same thing that happened in The Chernobyl Nuclear Power Plant Disaster happened to the largest nuclear power generator in the world, The Kashiwazaki-Kariwa Nuclear Power Plant, then in total about 2,246,742+ people would die
- The largest nuclear weapon ever tested, the 50-58 megaton Tsar Bomba that was dropped for testing on the uninhabited-at-the-time island of Novaya Zemlya, caused 500,000 deaths due to cancer caused by radiation from the bomb, and would cause in total about 11,877,807-12,448,057 deaths if it was detonated in modern Tokyo
- The largest nuclear weapon ever planned to be made, a 100 megaton Tsar Bomba, would cause in total about 14,488,869 deaths if it was detonated in modern Tokyo
And nuclear weapons are not the only man-made weapons to be made/planned to be made/thought of that can cause destruction at this scale. There are Super Guns, the V-3 Cannon, Sun Guns, Biological Weapons, man-made Black Holes, man-made Viruses and Super Intelligent robots, AI, nanotechnology, self-replicating machines and Grey Goo.