To me Theresia's threat to Guts was done to show that even if one does an objectively good thing, killing a literal monster, potentially saving so many people from torture and death, this could still seem as evil for some. It could hurt their person one way or another. In Theresia's case his father was brutally killed in front of her and she was left completely alone and helpless. I think you can make parallels, on a more massive scale, with the finial Guts vs Griffith battle. When Guts defeats Griffith, he isn't just gonna defeat a monster, but the savior of Midland and the rest of the world. He's gonna kill people's desired being. Even though we, as readers, know that Griffith is not the savior that the people see him as, but it's all the same for them. You could also find parallels with Guts vs Mozgus fight. There is no absolute good and evil, and every deed could be viewed differently depending on the perspective of the viewer.