Love is a powerful emotion, but does that matter if you refuse to acknowledge emotions?
It doesn't matter how little Seishirou wants to admit it, love directs his actions. If not for love, why did he make the bet? If not for love, why did Seishirou lose his eye? If it's not love, why is Subaru still alive?
Seishirou was raised to be a killer, acting only through his own will. He is unused to being unable to explain his actions to himself.
He is unused to being out of control. It would upset him, anger him, if he let himself feel anger.
He's no longer certain he has a choice.
Seishirou denies loving Subaru, denies it with all his (emotionless) heart. Furthermore, he refuses any claim on the Sumeragi except ownership. That denial will cost him his life, because all Subaru has ever wanted is to matter.
Seishirou will never let Subaru realize he matters. Seishirou Wishes he could care. Seishirou Wishes he could stop caring. Seishirou just wants simplicity. He'll never get that wish.
But it is a beautiful thing, to be killed by the person you love the most. If, that is, you admit to loving someone.