Even within Akaza's final moments, Keizo still guided him as a son and pupil, eventually welcoming him to the afterlife.
Even after being decapitated, Akaza managed to regrow his head through sheer willpower and his extraordinary regenerative abilities, continuing to attack Tanjiro and Giyu.
Rengoku and Akaza were reborn in the modern times and they first met in school. Akaza was alone mostly because his twin Hakuji was the popular one and was therefore mostly busy with extracurricular activities. Both twins are reserved, but Akaza is far more introverted.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation.
Because Akaza lost his will to live when his fiancee died. His last words to muzan were “I don't care..” as a human.
The real reason Akaza hates Doma
In fact, this is the only reason he can't stand Doma's existence. Akaza doesn't get along with any of his peers, but he is unusually hostile toward Doma since most of the latter's power originates from consuming women.
Giyu Tomioka Fulfilled His Promise to Protect Tanjiro
The Water Hashira, Giyu, survived the "Infinity Castle" arc after fighting beside Tanjiro against Akaza and Muzan. Though his sword was snapped by Akaza, and he lost an arm to Muzan, Giyu fought with Tanjiro and his fellow Hashira with everything he had.
Kokushibo was the final Upper-Rank to die during Demon Slayer's "Infinity Castle" arc. Muichiro, Genya, Sanemi Shinazugawa and Gyomei Himejima fought together against Kokushibo, though Muichiro and Genya were no match for him and were killed.
Post-Transformation
Sensing their despair, Doma offered a chance to him and turned both Gyutaro and Daki into demons.
Despite losing his memories, Akaza was noted to have subconsciously retained his strong feelings for Koyuki, this is seen where Doma noted that Akaza absolutely refused to kill and eat women during his hundreds of years as a Demon, despite his obsessive desire for more strength and power.
Doma is one of the most ruthless and insanely strong demons under Muzan Kibutsuji. As Upper Moon 2, he proves to be the absolute antithesis of Akaza. Moreover, their contrasting personalities as demons or mannerisms of rising up in the ranks are the major reasons for Akaza's hatred towards Doma.
Despite being a blood-thristy demon who has murdered several people, he has a set of principles he adheres to. One thing that Demon Slayer fans have noticed is that Akaza refuses to eat or kill women. Additionally, the Demon King, Muzan, gave Akaza special permission to do so.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
Akaza realized that his actions as a demon stemmed from the fact that he legitimately hated himself. Akaza subconsciously believed he was weak for never protecting the people he cared about. He couldn't properly process that he wasn't there to save the people he loved the most.
Since Nezuko is from a Sun Breathing lineage she can survive in daylight. After the final battle against Hantengu concludes she can step into daylight and many theories are available about the phenomenon. One of the theories is that her abstinence from consuming human blood is what saved her.
It has been said that no one has been able to kill an Upper Moon in a hundred years, since Upper Moon Six Daki and Gyutaro were killed.
Fortunately, Zenitsu was able to single handedly beat the Upper Moon 6. Not only did he defeat the demon, he also created a seventh form called the Flaming Thunder God. These are the demons who have been given the title of Upper Moon 6 in Demon Slayer.
The Wind Hashira has one of the saddest backstories in Demon Slayer. His mother had turned into a demon and had killed five of his siblings. He was forced to kill his own mother in an attempt to save his only remaining brother, Genya.
Giyu would still instruct Tanjiro to train to become a Demon Slayer with Urokodaki. As there is no Nezuko, Tanjiro would have no reason to pity demons, and he'll be much more harsh that he would be if Nezuko was alive.
When Tanjiro is presumed dead after the battle with Muzan Kibutsuji, Giyu cries and blames himself for not being able to protect him.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
Over a century ago, Akaza was a human by the name of Hakuji before being turned into a Demon personally by Muzan. He first appeared as the secondary antagonist of Mugen Train Arc, and grew to despise Tanjiro for humiliating him after Muzan reprimanded him for being unable to take his life.
He then started talking to Gyokko and complimented how beautiful his new pot looked. Talking about the pot that Gyokko gave to Doma, the latter said that he was using it to display a woman's head in his room. That is what infuriated Akaza, and he punched Doma hard enough to obliterate the lower half of his face.