See kids this is what happens when you don't learn to control your yandere. Via emotional manipulation and acting they can be quite tameable but dude never even tried.
@evilgrin is right that he could've also killed her. Pretty easily too. He just claim he's into her and wants to try bdsm, then once she's tied up then kill her. Personally I would've tried to learn to control her and uno reverse the situation, but hey if she's too unstable then what can you do.
Honestly it would've been even more interesting if instead of fighting her, he fell for her and had to learn to control her and clean up her messes so they don't get caught and he doesn't end up tied up again. "My sister is a murderous yandere but I love her" the manga. Lol.
I think fighting her is the biggest mistake, just causes more casualties. The only solution is joining her, if only to keep her under control for long enough to get rid of her.