I don't own Angel Sanctuary or Perfect Blue, this is a fanfic written for the 30kisses community on LJ, where I have claimed Kira/Katou. This was written for theme #15, Perfect Blue.
Spoilers for Perfect Blue.
It wasn't so much that Katou hated Setsuna. More the fact that he was jealous of him. He was positive if Setsuna and Kira hadn't been so close, he and Setsuna may have been able to become friends. That was never the case however, so he tried not to think too much about what may have happened, and focus on what did happen.
One of Katou's favourite movies was Perfect Blue. He wasn't exactly sure why, but there it was. Now, his favourite character wasn't Mima (though, she was hot and there wasa pussy or two. Sometimes he wondered if he hadn't bought the movie just for that, and then he realized that there tons of other movies with better pussy shots, so that probably wasn't it at all), it was Rumi.
He sympathized with Rumi. Mima had taken something from Rumi, and didn't even fully appreciate it.
Katou sometimes wondered if he could do the same thing to Mima as Rumi had. Take her life, drive her insane, and take back what was hers. Except replace hers with his, Mima with Setsuna, and Rumi with Katou. Steal a kiss from Kira (because he was certain that Setsuna and Kira were more than Just Friends if you know catch his drift. Then again, he had always been the jealous type, so maybe it was all in his imagination. He hoped it was, at least).
Of course, this plan was filled with dubiety. For one thing, it was Rumi who had ended up in the asylum at the end. For another, Katou wasn't quite that insane, and was sure he could win Kira back to him without copying Perfect blue.
Finally, he wasn't entirely sure that he wanted to hop on light poles in a pink tutu.