A decade ago, the anime and manga world was dominated by three series, and Bleach was one of them. It told the story of a teenage boy who could see ghosts, and he goes on to become a Soul Reaper who battles a myriad of spirits who are either good or evil. For a series to become as popular as Bleach, it needs to have a great villain, and Sosuke Aizen happens to be one of the best villains in all of anime.
Aizen's goal was simple: become immortal and take the Soul King's power by killing him. As the series progresses, fans get to see just how smart and powerful Aizen really is, and if wasn't for Ichigo, his plan might have succeeded. Aizen worked on his plan for more than a century, and during that time, he did some pretty despicable things that definitely crossed the line.
10 Betraying Momo
Momo may have not been the strongest Vice-Captain, but she was extremely loyal to Aizen when he was the Captain of Squad 5. In truth, Momo loved Aizen like a father, which is why she was overcome with grief when she found him dead.
Aizen was very much alive though, and when Momo sees him again, she is overjoyed. He knew how Momo felt, yet he deceived and exploited her nonetheless, and when the two embraced, he stabbed her through the chest with his Zanpakuto and left her to die.
9 Slaughtering The Central 46
Soul Society is unique in a lot of ways, and one of the things that really stands out is the fact that the Captain Commander does not have complete authority. That power lies with Central 46--an organization of wise men whose mandate comes directly from the Soul King.
Central 46 has complete judiciary authority, which is why Aizen secretly slaughters them all and sends out orders in their place. This was one of the smartest parts of Aizen's plan because he was able to speed up Rukia's execution to get his hands on the Hogyoku much faster. Doing this also left Soul Society in bureaucratic disarray for some time.
8 Using His Hogyoku On A Young Rangiku
When the Hogyoku was initially designed, it was intended to grant Hollow powers to Soul Reapers and vice versa, but its true power turned out to be the ability to materialize a person's deepest desire. There are two such items in existence, and Aizen created the first one, but in order to make it work, he fed it countless souls.
These souls belonged to Soul Reapers and innocent civilians. One of those civilians was a young Rangiku Matsumoto, the current Vice-Captain of Squad 10. This showed that Aizen was willing to kill children to get what he wanted.
7 Taking Barragan's Throne
Barragan arguably had the strongest Resurrección of all the Espada, but before joining Aizen's army, he was the undisputed King of Hueco Mundo. When Aizen appeared, he challenged Barragan and insulted him repeatedly, and when it was all said and done, Barragan was defeated and his entire army was slaughtered.
With Barragan's throne usurped, Aizen became the new King and transformed every powerful Hollow into an Arrancar. He did not treat his new subjects as individuals though. In fact, he basically enslaved them and turned them all into tools to further his plans.
6 Betraying Harribel
Tier Harribel was one of the more likable Espada, and she was a competent fighter who was extremely loyal to both Aizen and her Fracción. She was also the only Arrancar and Espada to not be defeated during the war in the Fake Karakura Town arc.
As his Arrancar army fell one by one, Aizen grew more and more frustrated by his minions' apparent lack of strength, which is why he decided to turn on them. He appeared before Harribel and attacked her without any remorse, despite the fact that she was still capable of fighting for him.
5 Faking His Death
Aizen's "death" was a turning point in the series because it showed that the Captains, despite all their power, could be killed. More importantly, though, it showed that Soul Society's highest ranking Soul Reapers had a lot of conflicting views and hidden animosities.
This perceived murder made every Captain and Vice-Captain question the loyalty of their counterparts, and it led to several battles that caused widespread panic and destruction. It also turned a teacher against his two prized students and pinned longtime friends against each other.
4 Creating White & Releasing It In The Real World
Ichigo possesses a lot of powerful abilities, and this is because his father was a Soul Reaper, while his mother was a Quincy. He also had Hollow powers, and this happened because his mother was bitten by a powerful Hollow who infected her soul and initialized the Hollowfication process.
This Hollow was called White, and it was created by Aizen as part of his research. White was created by fusing together the souls of several Soul Reapers, and aside from being cold and calculating, it was just as strong as a Captain. Aizen unleashed White on the human world, and it went on a rampage.
3 Setting Up Urahara
Kisuke Urahara started off as a simple shop owner, but as the series progresses, fans learn that he was once Soul Society's greatest scientist, as well as the former Captain of Squad 12. During the Turn Back the Pendulum arc, several high-ranking Soul Reapers were infected by Hollowfication as part of Aizen's research.
Urahara confronted Aizen, but he ends up escaping, and instead of going after him, Kisuke decides to help his comrades. He gets arrested the following morning, and he gets brought before Central 46 who finds him guilty immediately because Aizen used his illusion powers to give himself an alibi.
2 Creating The Visored
The Visored are interesting characters because they were all former Soul Reaper Captains and Vice-Captains who could all summon a Hollow mask and use Hollow powers. During the Turn Back the Pendulum arc, fans learn that Visored leader, Shinji Hirako was once the Captain of Squad 5, and that Aizen was his Vice-Captain.
At this point, Aizen's experiments led to the disappearance of a lot of souls, and this caused Squad 9's Captain and Vice-Captain to investigate, and when they turn up missing, Shinji leads a larger group to find them. It is here that they all succumb to Aizen's Hollowfication experiment, and Aizen then tries to kill his former Captain and comrades for being "failed experiments."
1 Making Hitsugaya Stab Momo
When Aizen faked his death, he made sure that Momo found a letter which identified Squad 10 Captain, Toshiro Hitsugaya as the likely perpetrator. This resulted in Momo confronting and attacking Toshiro with intent to kill, and this was terrible because the two were childhood friends.
They managed to repair their relationship to an extent as time passed, but Aizen made both of them suffer even more during the Fake Karakura Town arc. When several Captains attacked Aizen at once, he was caught in Toshiro's ice and was subsequently stabbed through the chest by him, but this was actually an illusion, and he forced Toshiro to impale Momo instead.