These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Ai no Kusabi
Angel/Devil Shipping: Many fanfic writers like to portray Riki as an innocent being seduced by the evil and perverted Iason Mink when Riki is not what one would call innocent.
Hate Dumb: Guy gets an extreme level of this from fans (usually Iason lovers) who have only seen the old OVA series, have not read the novel and do not really understand the how and why Guy is driven to do what he does to Riki. Many want him dead but also fail to acknowledge what he has done to Riki (physically harming a loved one out of love or otherwise), Iason is also guilty of. (The first time Iason penetrated Riki was rape and Riki never expected for things to go that far). However, since they haven't read the novel and the OVA leaves out most of the the abuse Riki suffered by Iason, they have a more sympathetic view of Iason and his actions over Guy.
It Was His Sled: The ending is a Twist Ending but due to the popularity of the first OVA series, the ending is better known than the actual details and dynamics of Riki and Iason's relationship and how things got to that point.
Magnificent Bastard: Iason, both magnificent and an utter bastard. He got his feisty sex slave to love him and die with him.
Iason really needs to be noted for his magnificence here because from the get-go, how he confined Riki, where he put Riki's pet ring and how he used it was to condition Riki to want him around. That's right, condition. That even when Riki wanted nothing more than to get away from Iason, he just wouldn't be able to live a normal life thereafter without Iason in it. Even letting Riki go back to Guy for a year was part of Iason's plan to show Riki that his old life was over no matter what and was very effective in proving that. After sex with an elite like Iason, it's hard to go back to a normal person like Guy. Really hard.
Memetic Sex God: Iason, who can cause orgasms with his voice alone and also undresses you with his eyes.
Moral Event Horizon: Kirie intentionally crossed it when he sold out Guy but did Guy do so when he decided to cut off Riki's penis to remove the Pet ring?
Nightmare Fuel: Both Iason and Riki are mutilated in the end of the story.
The Scrappy: A few may count given the ambiguous morals of the people of Amoi but Kirie and Guy stand out to the fandom.
Uncanny Valley: It's easy to forget that Iason is an Artificial Human, untill he rips Guy's arm off. Not to mention the time he made his manhood grow bigger in an enraged state of lust