A question for all the native speakers of english?

I'm translating an action movie. A bunch of bad guys are after a good guy who is trying to protect a scientist. The good guy asks them not to kill the scientist, but one bad guy does it with a single shot to the head. The bad guys says: "If you hadn't been here, I wouldn't have shot him. It's what kept our overhead down... One less bullet." I need hel with overhead. What does the whole phrase mean. Can you paraphrase the part from: It's what kept our overhead down. Thanks for clarifying this for me.