AN: This is my attempt to make sense of Seishirou's actions in TB and X, so here's him dealing with his highly-unhealthy-for-everyone-involved love for Subaru.
The truth was that no matter the outcome of their bet it would always end in tragedy. If Seishirou won Subaru would die, but if he lost it was a death sentence for the Sakurazukamori instead. The Sakurazukamori could only be killed by the person they loved, and they would, thus passing on the title.
If Seishirou won Subaru would die but if Subaru won he would eventually have to kill Seishirou. And Seishirou was selfish, how could he not be, growing up without being able to care about anyone but himself? So of course Seishirou was selfish even in love. He didn't want to kill Subaru even if the alternative would likely be worse for the 16 year old onmyoji. Subaru was so fragile and would no doubt completely break if he had to kill someone he cared about … well, if he killed anyone really. Not that he would ever kill anyone in the first place, at least not as he was now.
So when Seishirou realized he'd lost he had the new problem of figuring out how to get Subaru to hate him enough to want to kill him. It wasn't like he could just ask. The Sumeragi clan head definitely wouldn't be able to do it in the current situation, Seishirou had, after all, spent an entire year playing the kind, likable veterinarian and the effeminate onmyoji wouldn't kill a common murderer let alone someone he saw as a close friend.
No, if he wanted Subaru to kill him he would have to make it personal. The first thing to do was destroy the friendly relationship they'd built so he started with revealing who he really was, maybe he could break Subaru enough with just that, well it was worth a shot anyways. And break him it did but not in the way Seishirou had hoped, causing him to retreat into himself instead of lashing out at Seishirou. It was about what he'd suspected would happen but he had still hoped for something more violent. Showing everything he'd done and how he'd been deceiving everybody, lying about how Subaru had lost and was now going to die by his hand, hoping maybe to trigger any kind of self-preservation instincts, but no, Subaru didn't really care that much about himself did he? Seishirou's blank right eye proved that well enough. He wasn't actually going to do it of course, but unfortunately he had underestimated Subaru's complete lack of regard for himself.
Lady Sumeragi's timely intervention gave Seishirou an excuse to leave without revealing the inconsistencies between his claims and his actions. Besides, maybe he was a bit vindictive but the idea of what she'd had to sacrifice to "save" Subaru brought a small smile to his face, she wasn't the one he cared for after all and she had tried to keep the object of his (deeply twisted) affection from him.
He was fortunate that his next opportunity to make Subaru hate him came to him not long thereafter and without him needing to do anything. And she was even asking for it, having greatly misunderstood the Sakurazukamori's actions and motivations. For surely if anything was going to make Subaru want to kill him it was taking the life of his darling twin sister. He even had her permission to do it. And then with her last breaths she revealed the spell she'd cast upon him, not even realizing the perfect weapon she had given him. Subaru wouldn't even have to initiate the killing blow himself.
He did love how life worked out sometimes even if this whole falling in (obsessive and possessive) love thing was a bit annoying.
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