[ Oh that point. Sasuke remembers how pained the youkai became. Her spirit seemed to fade, losing hope for life the longer she was held captive. ]

I think the onmyouji confused his desire for the youkai for love. He doesn't love her. He wants ownership, to claim her as his prized possession. However, he calls it love to justify his actions.

[ The onmyouji might have been sympathetic at first, but he started to show his true colors. A person shouldn't hurt their lover the way he has. ]