A.N: everything is my Shinichi headcanon and I'm not sorry. Mostly I just wanted a story about him being horribly jealous of Shizuo to exist. So I wrote it myself.
It's not surprising that Izaya bugged Shizuo's apartment. Not out of any real desire to spy on his enemy (overall it's clear that Izaya doesn't consider Shizuo to be worth spying on) but to simply see if he could. Ultimately leaving the cameras and microphones in place as an option if a day ever comes when he needs to see what the monster is up to in his den.
Izaya doesn't use them, but the creature that calls himself Tsukumoya Shinichi does to a certain extent. It's another insight, a glimpse into the lives of the people that affect Ikebukuro the most. Certainly most people would view his actions as intrusive and wrong, but he will never see it that way. He's not the one who put the bugs and cameras there, he simply uses them. Another source of data to be compiled and sifted through. He doesn't really "see" as humans do, after all. Every camera is simply another pair of "eyes" for him, every microphone an "ear", senses that stretch through the city and around the world through cable and on wireless networks.
He is what he is, and knowing things is a primal instinct for him. Information is his lifeblood. It is what spawned him and what keeps him going and there will always be a desperate drive to learn more. If that means watching people in their most private of moments, then so be it. He will never use the knowledge he has against them, it's not his way.
But there will always be some facts that carry more weight than others even to something as impartial as he is. Ikebukuro and all its oddities is his fixation, the part of this great, wide, weird, world that is nearest and dearest to whatever he has that passes for a heart. Ikebukuro and its inhabitants he watches closely.
And Orihara Izaya he watches most closely of all.
Not that Izaya is really a part of that city, but he interferes enough to be. Izaya is sharp and dangerous and a witty conversationalist and he finds that these attributes are what make Izaya worth watching. His senses may spread across the world, he may be aware of everything that happens in Ikebukuro just as it is happening, but Izaya... Izaya he likes to actively watch and inevitably deal with on a one to one basis.
Which brings things back to the bugs in Heiwajima Shizuo's apartment.
They are something he has been aware of on a distant level for some time now. Something not quite worth any amount of attention because, quite simply, Shizuo's home life isn't a matter of interest to him. The interesting things in Shizuo's life happen outside of his home, when he is on the streets of Ikebukuro, when he finds himself inevitably tangled in the machinations of Orihara Izaya.
As such they aren't worth paying attention to... until the tangle that Izaya likes wrapping Shizuo up in follows him home.
Most would see it as a strange thing but he doesn't find it surprising. He has made a careful study of the way humans interact with one another, of the way these specific people act and react most of all. To him the prospect of Izaya managing to dig his way under Shizuo's skin deep enough for the pair of them to wind up rutting in Shizuo's apartment was never out of the realm of possibility. Granted, the calculated probability was only somewhere around three percent, but both Shizuo and Izaya are anomalous enough that they defy prediction more often than not.
He watches them that first night. The cameras and microphones Izaya had slipped into place all that time ago are high quality (Orihara Izaya will never use anything less than the best, even if he will ultimately wind up ignoring them) the video sharp and clear, the audio carrying every hissed curse and grunt to him. He is aware of it all.
When all is said and done he carefully stores a copy of all the footage in an isolated server he purchased early on in his awareness, even though he can't quite place exactly why he does such a thing. Certainly it's nothing he hasn't seen before. Sex is sex, all hormones and various chemicals in the brain humming around and bodies pressing against one another, sweat-slick and lost in the things minds and bodies make them feel. Other than varying degrees of enthusiasm (and which parts are involved as well as how creatively) one human coupling is very much like another from his perspective. There should be nothing special here. There should be no reason he found it as fascinating as he did.
He keeps it anyway. Studies it on occasion in a quiet, background, portion of his consciousness.
(and the moment when Izaya comes is a string of single frames viewed over and over again, eyes dark and needy and half closed, head thrown back, the pale line of his throat exposed and already bruised by lips and teeth, his own lips kiss-swollen and red and twisted into a strange sort of smirk, hands at Shizuo's throat even as his body grinds down onto the other man's cock...)
He tells himself it's a curiosity and nothing more.
The second time it happens does surprise him a bit. If the initial probability of Izaya and Shizuo "hooking up", so to speak, had been low than the probability of a repeat occurrence was infinitesimal.
But calculations of probability don't change the fact that it happens. And again he watches. Fixated.
Emotion isn't really something he considers himself capable of. He understands it as a concept, he is well aware of how it works just as he is aware of the fact that he lacks the physical reactions that are mostly associated with emotion. He is a free-floating consciousness; there is no real comparison between emotions as humans feel them and what he experiences.
As such, it's hard to name exactly what goes through his mind the second time he watches them. While he listens to their vicious banter (Izaya makes cracks about slumming, Shizuo threatens to snap Izaya's neck). While they twist and writhe on the floor because they don't make it to the bedroom at first. While Izaya lets out these keening little moans, his knees and elbows turning red from rubbing on the cheap carpeting. While Shizuo pounds into that slim body. While the pair of them grunt and moan like the animals that human beings ultimately are on some level.
He watches them, his mind a hum of observations and reactions. Reactions that he can't quite quantify even as he feels(?) them.
Hate. Jealousy. He understands these things as concepts.
He's not quite sure if that's what this is, even as he watches Shizuo (fuck) have sex with the man that draws so much of his attention.
Sex shouldn't interest him. There's no conceivable reason why he should want...
What?
(to make Izaya's face twist into that strange, warped, expression of bliss... just for him)
He just shouldn't.
He shouldn't watch them every time. There's nothing new to see.
He shouldn't keep saving copies of all of their encounters. One is so much like another there's no point to keeping variations.
He shouldn't keep studying them. Watching the way Izaya twists and moves and laughs and acts like he would happily fuck Shizuo to death if he could. Pent up aggression and hate bubbling out and spilling over into a vicious, twisted up mockery of what lovers do.
He shouldn't wonder what it would be like if it were him instead.
(they would be so much more cerebral, there would be cutting words from both sides and careful manoeuvring because the one to react first is the one that loses)
He shouldn't be thinking about any of these things. There are other things to watch besides Izaya. Other things to think about. Other things to do besides...
Besides quietly hate the man that has exactly what he wants.