Who/What Killed Sophie?

I finished The Razor's Edge this afternoon, which I started reading for my second, or probably third, time after A Doctor in the House and「劇画暮らし」. Was Sophie killed by zubrovka or by Isabel, or her own wantonness? "Do you know who did it?" "No, but I do. I think you killed her." "... You only lack only one thing to make you completely enchanting." She smiled and waited. "Tenderness."