The Theory of Happiness

The theory of happiness. It didnt make any sense. Why would anyone be so obsessed with something as simple as that?
Everyone in Yanagawa, Japan was obsessed with happiness at the time. Thus led to the case Akemi Taro had to find out.
Bodies, and bodies scattered the place. The killer would leave behind notes. Different variations of happiness. The definition. The theories of happiness itself. Smile faces, laughter, fun. It was all this killer wrote about.
Akemi was tired of it. He hated seeing the bodies lying around like this. The crime rates were getting higher. It felt like no one was safe. Until, he had gotten an anonymous letter.
The letter had asked him to join him at an old storage facility. When he went, he came to find Kaori Ruto, also known as "Seven".
Seven was known for his acts of crime, he had to go to a mental hospital for months. Yet, the infamous killer shocked the detective, offering his help on the case.

Warnings:

Blood/gore
Cursing
Abuse
Suicide/self harm
Hate against/towards religion(s)
LGBT+ (if thats even a warning? i dont think so but just in case: this story's kinda gay in case you guys dont like that shit i dunno)