Guts used the Count's emotions to his advantage because he knew that deep down, he is still a human. He was aware the he'd never dare kill his daughter so she got used as a human shield.
The Count's pleas may make it look like pitiful overkill, but this monster has tortured hundreds of innocents without remorse and almost killed the person who is currently torturing him. I'm sure Theresia saw her dad's fate coming the moment he smashed Guts into a pillar kek
Guts holding his sword with his mouth and slicing that fiend's head was awesome af~
Damn. The GodHand has arrived...