“Thomas Midgley Jr. He played a major role in development of tetraethyllead ( leaded gasoline ) which went into mass production throughout the 1920’s. This invention meant that the blood lead content in developing children rose causing numerous problems, including cognitive issues and physical growth, it was also noticed that the rise in blood lead content matched with violence in adolescents in multiple countries.

Midgley is also accountable for the discovery of chlorofluorocarbons (CFC) specifically Freon which as many know had a drastic impact on the Ozone. As of October 2020 the Ozone hole above Antarctica sits approximately 23 million square kilometres /8.9 million miles. Although, these 2 inventions seem insignificant compared to more drastic actions taken by other single humans its the widespread impacts that his inventions had and are still being felt today. Even though both, leaded gasoline and CFC’s, have been banned.” - nat3dog3

“Trofim Lysenko. A Soviet biologist who thought Gregor Mendel was a reactionary whose thoughts had no place in the Soviet Union. He tried to reinvent agricultural practices to maximize productivity, having farmers put seeds very close together under his belief that plants who belong to the same “class” will not compete for nutrients.

He was given the Order of Lenin eight times for his pseudoscience, while his “theories” contributed to the deaths of millions through famines that plagued the USSR. Later he was finally discredited after Stalin’s death, but then Mao Zedong adopted his practices, which helped cause the Great Chinese Famine” - Star_Trekker

“Guy de Chauliac. He was a surgeon in the 1300s who vehemently spoke out against another fellow surgeon, Theodoric Borgognoni. Theodoric was a surgeon who wrote about his theories on proper wound care and believed that the best thing you can do to a wound is wrap it and keep it clean. Guy hated what Theodoric was writing because it directly went against the teachings of Galen, an Ancient Greek surgeon who believed pus was the body’s way of balancing your humors. Guy’s teachings were widely accepted and it’s believed that his ignorance set the development of antisepsis in surgery back about 600 years.” - better_than_shane

“The Mongolians, army of Hulagu, when they destroyed Baghdad's libraries that contained many of the past knowledge that setback humanity years.” - OctupussPrime

"Purportedly, the unnamed inventor of flexible glass (vitrum flexile) brought a drinking bowl made of the material before Tiberius Caesar. The bowl was put through a test to break it, but it merely dented, rather than shattering. The inventor repaired the bowl easily with a small hammer, according to Petronius. After the inventor swore that he was the only man alive who knew the manufacturing technique, Tiberius had the man executed. He feared that the glass would devalue gold and silver, since the material might be more valuable." - catebriga

“I would suggest Andrew Wakefield. His work "Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children" has been directly responsible for the mistrust of vaccines, the decline of Western rates of vaccination, and basically telling everybody that there's a boogeyman underneath your bed. He is directly responsible for thousands and thousands of deaths.

"He's a scientist who made a mistake! Why are you blaming him?!" It's not that he made a mistake. He had purposely customized the outcomes to meet the hypothesis so that he could get rich selling "safe" vaccines and diagnostic kits. He poisoned the well that saved so many people just so he could get rich. I have no idea how he can sleep at night.” - danfromwaterloo

“Shayk Al-Islam. I heard of this guy after hearing someone on TV complain about how this man set the Islamic world back by centuries. In 1515, the age of the Ottoman Empire, he, a “learned scholar” of the kingdom, issued a decree that forbid printing (press) and made using it punishable by death.” - naman_is

“Yongle Emperor of the Ming Dynasty in China ordered the fleet of Zheng He, the greatest trading and exploration fleet of the time, to be burned during his reign in the early 1400’s. This was the beginning of an era of isolation for Chinese kingdoms, which ultimately lead to the collapse of imperial China, and indirectly to the rise of the PRC.

Additionally, the wealth of the world overall decreased as a result of reduced trade with China, and if China had continued exploring it is possible that they, not Europeans, would have colonized North America (instead of merely maybe discovering it then telling no one as they did in history). It may not be a significant alteration of human progress, but it’s one of those events that sets the world in a definitively different direction.” - nobd7987

“Bakhtiyar Khilji. Hands down. In 12th century there was the world's biggest university in India named "Nalanda" where intellectuals from all around the world used to study. Then Turks invaded India under Khilji. They killed almost all the intellectuals and destroyed the university. And they BURNT the library. The library continued to burn for 3 MONTHS. This has to be by far the biggest loss to mankind imo.” - SorcererSupreme13

“Sir Mark Sykes. This man was the British element in the Sykes-Picot agreement. For those of you not in the know, 100 years ago the Middle-East was an area that did have some nations and some more tribal areas. So people were more divided by language and culture, some by religion. France and Britain decided to carve up the Middle-East into easier to govern territories, but fumbled this task and instead divided the territories on the map OVER these religious and tribal lines.

Not only has this been a main contributing cause of conflict in the Middle-East (if you take two opposing or rival groups and then suddenly group them as one country, what do you expect...) but said conflicts have then fueled further conflicts again and again. This has then been further used by Islamic extremists as a reason to hate the western powers, as they were the ones who created this terrible agreement. Even Sykes himself accepted that the agreement’s wording should be changed in order to give those countries autonomous rule.

What is a little sad is he actually seemed to want to help these regions with the agreement, but just bumbled the whole thing which has led to most of the issues the Middle-East has to this day. Sykes didn't make the modern Middle-East though, he just played a large part in creating the circumstances in which its current problems thrive. Imagine all of the advancement, education and collaboration that could have happened had the Middle-East been allowed to flourish unhindered and without resentment?” - Lit-Rature

“Edward Bernays. He created modern day PR, which set the blueprint on how to control the masses by deception.” - invisiblepeep

“Cyril of Alexandria is a pretty good bet. He organized murder squads to go around Alexandria killing anyone who wasn't a die hard Christian. This included nearly every great mind of the era. One in particular was Hypatia of Alexandria one of the greatest minds in history. He is of coarse considered a saint by Catholicism and the church of England…” - zenivinez

“Who ever gave the order to burn the library of Alexandrea.” - LeverTech

“The Exxon scientists who first learned of man made climate change in the late 70s and chose to cover it up.” - boarderwhoskis

“Up there would be Diego de Landa, who burned all of the Mayan books in the name of Christianity, on July 12th, 1561.” - astoundingSandwich

“Nasir the copper merchant. Our first written texts could have been amazing accounts of how life was at the time. Instead they're just people giving bad reviews on bronze age amazon.” - Largebluntobject

“Queen Victoria, she was responsible for the deaths of around 60 million people in Brazil, China, India, and Ireland. In all of these cases the crown participated in the exporting of goods from famine struck areas, and waged campaigns to make poverty worse.” - Not-the-batman

“Whoever seized all of Nikola Tesla's chests and papers after he died.” - Thereisnopurpose12

“In terms of sheer devastation, arguably Genghis Khan... estimated to have killed about 11% of the world's population in a very short period of time. Would be like killing 800 million people today. You could argue though it didn't set the world back THAT much, given most of the important technological developments happened in last couple of centuries.

That society hadn't progressed that much from Genghis Khan to 1,000 years later. So you might wanna refine what you mean by 'progression of humanity'. Arguably, that'd be whoever delayed those basic medicines/industrial revolution the most... and in that case, some smug bastard we probably never really knew about who junked a bunch of important findings.” - roymondous

“Charles Babbage. It wasn’t necessarily malicious, but he hindered progress of computers for around 100 years. In the 1800s (industrial revolution) he designed a working computer. He had interested buyers. He had financial backwards. He had created designs for analytical engines that wouldn’t be created until the next century.

And what did he do with all of this? Nothing. No final product was manufactured or released, and we didn’t end up with computers until the 1900s. Imagine what we’d have today with an extra 100 years under our belts in computer development.” - Captain_Cookiez