The episode marked the final regular performances of actors David Costabile and Jeremiah Bitsui as Gale and Victor. Gilligan considered changing the episode based on audience responses to the third-season finale, "Full Measure", on whether Gale would be killed or not, but ultimately decided against it. During a twist scene in "Box Cutter", Gus slices his loyal henchman Victor's throat with a utility knife right before Walt and Jesse's eyes. Esposito said he was concerned about filming the scene "without really hurting my spirit and my soul", and he concentrated so hard that he remained silent and focused during filming, rarely speaking to others on set. The scene was so bloody it made Cranston's daughter faint during a screening.

Who did gus kill with a box cutter? OK. Let's think. My stream of consciousness:
To answer the question, consider the following: During a twist scene in "Box Cutter", Gus slices his loyal henchman Victor's throat with a utility knife right before Walt and Jesse's eyes. The answer is his loyal henchman Victor.