Pivotal Moments: The Homocide of Gokurakuji Station

The murder of Jyugo's cousin Yuka by Kuu's hands speak volumes of his descent into madness, despair, and hatred. Unlike the deaths of the children in the orphanage and the first murders of the festival-goers preceding the train murders, these killings were not spur-of-the-moment crimes of passion brought on by unspeakable despair. Although Kuu found himself overcome by his emotions and the feelings of betrayal, the murder of Yuka was calculated, premeditated, and acted out in a warped sense of cold-blooded payback.
Kuu bid his time well. Rather than attack at the station as Jyugo said goodbye to Yuka and her mother for what they assumed was only another year, he waited and watched. Rather than kill Jyugo after Yuka and her mother boarded the train, Kuu snuck onboard the train himself and waited in the next cabin. Only after Yuka's fight with her mother did he finally make his presence known, and while Jyugo was happily surprised to see him through the window, Yuka feared for their lives from the moment she saw him. Jyugo entered the train, just to say hello to Kuu. Her pleas for her cousin to run away fell on deaf ears, as Jyugo refused to believe his friend could be responsible for the deaths at the festival. As Jyugo told his cousin he hated her for lying to him; Kuu killed the girl by cutting her in half at the waist. As Jyugo screamed and Yuka's mother cradled the brutally maimed corpse of her child, Kuu merely laughed, saying Jyugo was the stupid one and not his cousin. When Yuka's mother turned her attention toward him, perhaps meaning to scream at him, he killed her instantly by beheading her. In shock, Jyugo asked him why he would do this to him. After all, weren't they friends? The boy's answer was that they were friends, and their friendship was the only reason Jyugo still lived. As Kuu walked past him, he stated he would be his next prey, prompting Jyugo to tackle him, shake him, and beg him to stop. As he pleaded with him to cease his bloodshed, Kuu wondered how it came to this. He knew that all he'd wanted was Jyugo's affection, yet he'd caused him so much heartbreak. He tore a hole in the roof of the train to escape, leaving Jyugo alone with the corpses of his family.
Kuu proves through these murders that he is every bit the slave to his emotions that all children tend to be. Children, like adults, possess logic and reasoning, but due to their lack of experience, such logic and reasoning tends to revolve around themselves. Rather than consider what is right or wrong from an altruistic standpoint, they believe what is right to be based on their wishes and whether or not the actions done to them hurt them or help them. In spite of his upbringing, or lack thereof, teaching him to guard his emotions so they won't be abused by others, he easily gives in to callous vengeance over a white lie. On the bus ride home, he wonders what he would do if Jyugo's cousin was indeed a girl. If he wouldn't go to the festival with her but with another girl instead, his feelings would be hurt, and to a child's mind, the best way to handle being hurt? Often, it is to hurt the one who caused it. The DNA Voice exacerbated the situation by suggesting it was the right thing to do, and Kuu was already so far gone into what it suggested that he attempted to strangle Jyugo for the slight he hadn't yet committed before he realized what he was doing. Immediately after the murders, his coldness toward Jyugo as he grieves and asks him why he killed his cousin and her mother is that of a child scorned. He's ignored his feelings by lying to him, but now he'll make him suffer for it. And what better way to make him suffer more than to go and kill the girl he lied about in the first place? It would also free him of his ties to the world. Without Jyugo or his family in his way, he could go about creating a place for himself where he wouldn't ever be hurt by other people again, where he would be surrounded with people like him who cared about him and wouldn't treat him as badly as those he's met thus far.
Like many young special children, Kuu had no idea of the full scope of his powers when they first developed, nor did he grasp the consequences of using such power. He first used them in a moment of passion, killing the children who not only bullied him but murdered his pet. After that, he used them to kill others so he could use their homes as shelters, and he did so without truly grasping the severity of what it meant. It was just a means to food and a warm bed at night, things he couldn't have since he was on the run for the mayhem in the orphanage. The idea that what he was doing was wrong came to her during the trip with Jyugo to the zoo, but it was swiftly forgotten as the two had a fun and carefree day Kuu himself never got to experience before. This realization didn't come up again until after the murders when he realized too late that, in seeking to make Jyugo pay for lying to him, he ruined his life completely. Other special children likely reached similar epiphanies after killing their own families by accident; those they killed with their amazing powers wouldn't come back, even if they didn't mean to hurt them.
The murders of Jyugo's relatives are examples of how far Kuu fell into his destructive feelings, as well as the tipping point for him becoming a mass murderer. They are the sign to Kuu that his choices and his actions have weight, as well as signs that he is not always in the right, but at the same time, he concludes from them that being a killer is just what he was born to do. After the murders, he takes to Kamakura's streets and continues to kill people, either to take their homes for a few nights, or simply killing them just for the sake of it. While mourning his actions against Jyugo, he does nothing to actively stop them when geared toward other people. Once more, killing becomes a way to have food and a place to sleep that isn't the streets, with only the occasional lament that he's fallen into the pit of killing in cold blood.
The murders also lead him to be more careful when he meets another friend in the future, and a possible chance at redeeming himself and proving he isn't solely a heartless killer. However, despite his efforts to do right by his new friend and try to correct himself, things do not go as he plans. The murders also lead to his arc of atonement in Nanbaka's main plot in both the manga and anime, as Kuu's reasons for living are vengeance for Aiko and apologizing to Jyugo for what he's done.