In Bleach, fans are exposed to several moral dilemmas, many different shades of good and evil, justice and villainy. Soul Reapers against Hollows and Quincies, and all the unique characters presented in each faction showcase Tite Kubo's eloquent writing when it comes to the different shades of gray people come in. There's no pure good or pure evil. Therefore, it is unrealistic to portray such characters when you want a deep story. But the Soul Reapers are a good look at how dark the supposed heroes of the story can be.
The Dark Beginnings of The Gotei 13
In the ranks of the Gotei was a woman who took the name Yachiru Unohana. Unohana was an extremely skilled swordsman, but she was also cold and detached from the value of life and lived for an unusual purpose, the thrill of battle. Unohana lived to kill any worthy warrior in her path, and she wasn't the only one in the Gotei who would just trample over lives.
A race of supernaturally gifted humans called the Quincy emerged and started a war on the Hollows. The Quincy erased the Hollows from existence, completely throwing the balance of souls out of orbit. And so, the Gotei 13 declared war on them, and with the full might of thirteen powerful killers, they massacred the Quincy and their king, Yhwach, driving them almost to extinction.
Gotei 13 and Central 46, Corrupted Justice
Kaname had a very close friend he cherished with all his heart, a kind woman married to a member of the Gotei 13. This Soul Reaper belonged to one of the noble families. One day, Kaname's friend was killed by her husband, and he begged for justice. He begged for the arrest of this Soul Reaper, but the man walked away without any punishment, as his status as a noble protected him from harm. This event would send Kaname onto a path of vengeance that would lead him to join Sosuke Aizen, betraying the Soul Society.
(The following contains spoilers for the Thousand Year Blood War)
The Soul King
The Soul King was mutilated and dismembered, and his body was used to separate the four realms and hold them together in harmony. The Human World, Soul Society, Hueco Mundo, and Hell, are all held together by the imprisoned Soul King, a crystallized corpse at the height of the Soul Society.
The Soul King has no real agency over the world and only exists merely to maintain the balance of lives. He's far from a ruler, he's the Soul Reaper's ultimate sin, as they turned the creator of the universe into a living lynchpin for all existence.
Whether the Soul Reapers are truly the good guys of the story is all up for debate, but what is undeniable is that besides all the darkness that envelops the Soul Society, there is also great kindness and heroism; captains like Sajin Komamura, Juushirou Ukitake, Toushirou Hitsugaya and the lieutenants Rukia Kuchiki, Renji Abarai and Rangiku Matsumoto. The Gotei 13 are a very morally gray group as the protagonists of Bleach.