A lot of this is going to rest on your specific moral views and the way you define the question, rather than historical factual issues, I suspect.
The clearest examples are going to be things like genocides and wars. For example, the An Lushan Rebellion was (arguably?) responsible for the largest fraction of humanity to be killed in on event. There have been enough of these that you could fill a very respectable list.
If you relax your definition of what constitutes an event, things like ‘all murder ever’ are going to qualify. But these are clearly less dense in space-time than genocides, and many people would not count them as ‘an event’.
If you accept events that are less bad per capita, but more widespread, things like the denial of education to women, which has been quite common, and could plausibly qualify.
If you think that government acts should be judged similarly to private ones, mass taxation, conscription, imprisonment and immigration restrictions might qualify.
If you accept more indirect responsibility, the failure to accelerate technological development resulted in hundreds of millions of unnecessary deaths from disease.
If you believe in a right to a competent electorate, voters supporting predictably incompetent or immoral governments could qualify.
If you accept ex ante rather than ex post catastrophes, early nuclear testing, where there was a plausible risk the atmosphere might catch fire, could qualify, though maybe the responsibility is not widespread enough.
If you accept victims other than adult humans, things like factory farming and abortion will qualify.
If you assign a lot of weight to children’s preferences, mandatory schooling which forces them into strict compliance for most of the day could qualify.
Abortion is only a moral catastrophe if you reject antinatalism. From an antinatalist/negative utilitarian perspective, one could argue that abortion prevents an entire lifetime worth of suffering. This is especially the case if abortion disproportionately targets fetuses that would have lived lives that are worse than average.
A lot of this is going to rest on your specific moral views and the way you define the question, rather than historical factual issues, I suspect.
The clearest examples are going to be things like genocides and wars. For example, the An Lushan Rebellion was (arguably?) responsible for the largest fraction of humanity to be killed in on event. There have been enough of these that you could fill a very respectable list.
If you relax your definition of what constitutes an event, things like ‘all murder ever’ are going to qualify. But these are clearly less dense in space-time than genocides, and many people would not count them as ‘an event’.
If you accept events that are less bad per capita, but more widespread, things like the denial of education to women, which has been quite common, and could plausibly qualify.
If you think that government acts should be judged similarly to private ones, mass taxation, conscription, imprisonment and immigration restrictions might qualify.
If you accept more indirect responsibility, the failure to accelerate technological development resulted in hundreds of millions of unnecessary deaths from disease.
If you believe in a right to a competent electorate, voters supporting predictably incompetent or immoral governments could qualify.
If you accept ex ante rather than ex post catastrophes, early nuclear testing, where there was a plausible risk the atmosphere might catch fire, could qualify, though maybe the responsibility is not widespread enough.
If you accept victims other than adult humans, things like factory farming and abortion will qualify.
If you assign a lot of weight to children’s preferences, mandatory schooling which forces them into strict compliance for most of the day could qualify.
Abortion is only a moral catastrophe if you reject antinatalism. From an antinatalist/negative utilitarian perspective, one could argue that abortion prevents an entire lifetime worth of suffering. This is especially the case if abortion disproportionately targets fetuses that would have lived lives that are worse than average.