I'm not certain it was that torture that aroused Farnese, rather I think it was just being alone with a dangerous hunk of a man like Guts. Coming from a seemingly sheltered and devout upbringing, I doubt she's ever been with a man, much less see someone like Guts half-naked. After letting her composure go by whaling on Guts, a part of her took over, the more animalistic side she tries to suppress through prayer. Giving in to her momentary ever-suppressed feeling of lust. But maybe she actually is a sadist, lol idk, I guess we'll see.
It's crazy how I almost forgot about the demons haunting Guts at night after having all this new story development. He's already captured, stripped down in a cage and now the demons are out to get him. The man can never catch a break but that's why I love him - cause even then he refuses to lay down!
Very captivating way to end Vol 16. Looking forward.