History has a way of shaping the future, one such example of this is people as a whole, learning from their mistakes and not making them again in the future. As the old saying goes, those who do not know their history are bound to repeat it.

There's no question that the historical events on this list were awful, but they also led to what was arguably a much greater good.

Fritz Haber is credited with creating the Haber process. Which is used to create ammonia. He developed it so Germany had a better supply of gunpowder and other explosives during the first world war. If not for him, Germany wouldn't have lasted as long as they did. Effectively lengthening the war.

However, another excellent use for ammonia is fertilizer. Although Fritz never intended for it, he's also the reason millions of people never starved to death.

Dinosaurs had to go for mammals to rise up as the dominant order on Earth.

Religion. Definitely needed it a few millennia ago, now it's just holding back progress.

Most wars lead to technological and medical advancements that likely never would have occurred without war.

Taxes

Dave Chappelle spoke on this but the murder of Emmet Till - more specifically his open-casket funeral. No human deserves what happened to that boy but the aftermath of those images being published sparked a lot of change.

US Civil War. Slavery was not going away without it.

The Crimes of Ted Bundy. I know, a weird one. He taught us a man who looks ahem "hot" and studies a prestigious degree can also be a horrible person and do vile things to others.

He revolutionized forensic psychology and police investigations. He helped psychologists build the blueprint for the modern psychopath.

Allying with Joseph Stalin in WW2.

Whatever the f--k was going on with Victorian psychological medicine. That kind of stuff was barbaric by today's standards but back then, even the creepiest and most brutal stuff was paving the way towards modern medicine. It was the first time they actually started caring for mental patients, instead of giving them to the church or streets. They experimented, learned, and tried.

The Black Plague. During the time before the outbreak, the world lived in pretty close together city dwellings. Like several families to a room. Most of Europe was a feudalistic state so no one could own anything like land or themselves for that matter. The rivers were toxic from people just throwing feces and other vile things just anywhere.

After the plague ended most of Europe had died. Feudalism was ending rapidly. Wages went up and so did life expectancy. People spread out and developed better sanitation. The ground was [richer] with nutrients for growing crops on account of the mass graves.

WW2 is controversial but it was the main factor why there has never been a major war since.

To this day we experiment on rats. We give them Cancer, HIV, we get them addicted to drugs, we amputate their limbs, etc., etc., etc.

And it’s saved hundreds of millions of human lives. It’s... troublesome. It’s one of the hardest ethical conundrums for me. Because I literally use a medication every day that drastically improves my life quality, and it wouldn’t exist if not for all those rats which were experimented on.

Don’t know if this has been said and it’s kinda hyper-specific, but Churchill and the siege of Calais. He ordered the garrison to hold to the last man, effectively sacrificing a few thousand lives as there was no evacuation order planned. The battle would tie up German forces so the evacuation at Dunkirk could proceed. A few thousand vs. roughly 300 thousand.

Body snatching in the 19th century.

Medical schools would illegally buy human bodies that were dug up from graves in order to study anatomy and teach medical students in secret because at the time it was illegal and not acceptable to use human bodies for Science or education purposes.

The only way schools could get human bodies legally was from death-sentenced people who also had been condemned to dissection on top of that by the courts, which was very rare and couldn't supply enough bodies for Medicine schools.

So there was a literal black market of dead bodies and the body snatchers would risk the death penalty themselves to make a profit from digging up bodies that were freshly buried. The bodies of young adult people were particularly sought after and expensive.

But it also allowed scientists and students to learn about anatomy and improve surgery techniques, which allowed Medicine as a whole to make a lot of progress and saved many lives in the end.

The oxygen-emitting cyanobacteria wiping out most life on earth 2.5 billion years ago.

Allowing some attacks to take place even after Hitlers Enigma encryption device was cracked by the English and they knew when and where the attacks would occur. They could only act on the intelligence to thwart attacks if they could quickly come up with an alternate way to find out which the Germans would find believable.