Levi had this part down to an art. If she turned on him, did anything, he would strike and his aim would be perfect. Levi had no issue killing, but he didn't like doing it to humans. He was more accustomed to killing other things. Things that hurt humans.
His attention shifted to Johanna as she answered his question, and with a laugh. His eyes narrowed briefly. "I see. Yes, so you killed to survive. I understand. I'm a soldier myself, but I didn't kill humans. I fought monsters who sought to invade my world and destroy everyone in front of us." Levi clenched a fist, then gave the girl a look. "It's why I had hoped that we could return home."