Kaido is one of the Yonko of the Sea and the main villain of the Wano Country arc in One Piece. Before starting his own crew, he used to be a member of the legendary Rocks Pirates. After the God Valley incident, Kaido and the rest of the crew disbanded. With time, he managed to make a name for himself with his strength.
Eventually, he rose to the status of Yonko, and he was given the moniker of "World's Strongest Creature." With an unimaginable amount of strength, there are only a few One Piece characters who can oppose him. So, Kaido is allowed to do whatever he wishes, and considering his evil nature, his actions usually end up harming other One Piece characters.
7 Boiling Oden Alive
After taking over Wano, Kaido began to rule it with Orochi's help. Once Oden returned to Wano, he was shocked to learn that his beloved country had fallen prey to Kaido. Oden tried to confront the two usurpers, but they blackmailed him into dancing naked in the streets of Wano. To the people of Wano, it seemed Oden had lost his mind, and eventually, they started to lose respect for him.
After putting up with the demands of Kaido and Orochi for several years, Oden finally decided enough was enough. So, he set forth to battle Kaido and his underlings. For a samurai of Oden's caliber, defeating Kaido wasn't difficult. Unfortunately, before Oden could finish Kaido off, he was distracted by Koruzumi Higurashi. This allowed Kaido to land a critical blow. Oden and his retainers were captured and sentenced to being boiled alive for their transgressions.
6 Turning Wano Into A Wasteland
Before Kaido started to rule Wano, it used to be a beautiful country that was filled with life. Most of the people had food and clean drinking water, which was enough to get by. Under Kaido's rule, everything changed. He opened up multiple armament factories that created weapons for his army.
These factories produced toxic waste which was dumped into the water bodies, making the water unfit for drinking. There was a major shortage of food, and countless people succumbed to hunger and died.
5 Trying To Kill Momonosuke
Momonosuke is the eldest child of Kozuki Oden and Toki, which made him the legitimate heir to the shogunate of Wano. After Oden's death, Momonosuke was the first in line to be the shogun. So, Kaido considered him a threat and tried to eliminate him multiple times.
During the Fire Festival, Kaido came extremely close to killing Momonosuke, and if he had succeeded in his task then the alliance would have completely fallen apart. What's surprising is that Kaido had absolutely no qualms about killing a child if it meant that he would retain his authority. Luckily, Sanji managed to rescue Momonosuke before the latter could be executed by the Beasts Pirates.
4 Beating Up His Own Crew Members
Kaido and the other higher-ranked officers of the Beasts Pirates are apathetic. They do not care about their underlings, and they do not hesitate to attack them if they get in the way. On many occasions, Kaido has injured his crew members in his drunken stupor.
To Kaido, any weakling is dispensable, which is a big reason why he treated the other crew members horribly. In comparison to the other legendary pirates like Whitebeard and Roger, Kaido is a heartless monster.
3 Forcefully Feeding SMILEs
Despite being a brute, Kaido has his own ideas about peace and equality. As someone who was involved in wars from an early age, Kaido firmly believed that people could only be equal in war. Kaido's abrasive nature, combined with his urge to have a large-scale war, made him a scary character. Kaido wanted to raise the mightiest army, so he decided to build a crew full of devil fruit users. However, there was a serious problem with his plan: getting devil fruits was not easy.
To tackle this problem, Kaido ordered Doflamingo to come up with a solution. Doflamingo formed a deal with Caesar, and together they created SMILEs for Kaido. SMILEs are artificial devil fruits that have severe side effects. If a person eats a faulty SMILE, they lose their ability to swim, and they can't express any emotion other than happiness. When Kaido captured Eustass Kid and Killer, he forced the latter to consume a failed SMILE, which robbed Killer of his ability to express his emotions properly. The fact that Kaido has forced other people to eat a SMILE against their will shows how cruel he is.
2 Imprisoning Yamato
Yamato is Kaido's son, who has been inspired by Kozuki Oden. As Oden's biggest fan, Yamato questioned his father's methods. Kaido did not like his son's behavior, so he decided to imprison him.
It was another showing of Kaido's heartlessness, as it did not cross his mind that Yamato might be traumatized for life. Luckily, Yamato turned out to be an incredible person, and even helped Luffy to defeat Kaido.
1 Helping Orochi Stay In Power
Orochi was an evil and selfish character, who only cared about overthrowing the Kozuki family and bringing the Kurozumi back into power. He plotted his revenge for several years and eventually, with Kaido's help, he became the shogun of Wano.
Orochi led a lavish life, and he had no consideration for his subjects. Anyone who dared to question his authority was sent to the gallows. Without Kaido's continued support, Orochi would have never managed to usurp the Kozuki clan. By extending his aid to Orochi, Kaido showed that he was truly an evil villain.