Let’s define a state-led mass killing as an episode in which state security forces or groups acting at their behest deliberately kill at least 1,000 noncombatant civilians from a discrete group in a relatively short period of time—weeks, months, or maybe even several years. This is a paraphrased version of the definition my colleague Ben Valentino developed for a U.S. government-funded research project, so using it allows us to identify and compare many episodes over time, as I did in another recent post.

Since World War II, nearly all of the state-led mass killings that have occurred around the world have followed one of three basic scenarios, all of them involving apparent threats to rulers’ power.

First and most common, state security forces fighting an insurgency or locked in a civil war kill large numbers of civilians whom they accuse of supporting their rivals, or sometimes just kill indiscriminately. The genocide in Guatemala is an archetypal example of this scenario. In some cases, like Rwanda, the state also enlists militias or even civilians to assist in that killing.

Second, rulers confronting budding threats to their power—usually a nonviolent popular uprising or coup plot—violently repress and attack their challengers in an attempt to quash the apparent threat. The anti-communist massacres in Indonesia in 1965-1966 fit this pattern. In rare cases, like North Korea today, just the possibility of such a threat suffices to draw the state into killing large numbers of civilians. More often, state repression of nonviolent uprisings succeeds in quashing the challenge with fewer than 1,000 civilian deaths, as happened in China in 1989, Uzbekistan in 2005, and Burma in 2007.

Third, rulers who have recently seized power by coup or revolution sometimes kill large numbers of civilian supporters of the faction they have just replaced as part of their efforts to consolidate their power. The mass killings carried out by the Khmer Rouge in Cambodia in the late 1970s are probably the most extreme example of this scenario, but Argentina’s “dirty war” and the long-running political purges that began in several East European countries after World War II also fit the pattern.

What happened in Egypt yesterday looks like a slide into the third scenario. Weeks after a military coup toppled Mohamed Morsi, state security forces violently assaulted crowds using nonviolent action to protest the coup and demand Morsi’s restoration to the presidency. The death toll from yesterday’s ruthless repression has already surpassed 500 and seems likely to rise further as more of the wounded die and security forces continue to repress further attempts at resistance and defiance. What’s more, the atrocities of the past 24 hours come on top of the killings of scores if not hundreds of Brotherhood supporters around the country over the past several weeks (see this spreadsheet maintained by The Guardian for details).

One of the many rationalizations offered for the July 3 coup was the argument that the Muslim Brotherhood had used violence to suppress its political rivals during and after mass protests against Morsi last December. People were right to challenge the Muslim Brotherhood over that thuggery, which was arguably a nascent version of the second scenario described above. In calling on the military to deliver them from that threat, however, some of those challengers seem to have struck a Faustian bargain that is now producing killings on a much grander scale.