If the rock is the size of your starter rock in Rust, sure a headshot would do it. But a baseball sized rock, unless it's flung from a seasoned pitcher, a headshot won't kill a zed. It's just damaging their skin and won't bruise. Rocks rarely kill humans unless it's a pound+/headshot/with their head against an immoveable object, so why would it kill a zed who feels no pain/trauma/concussion?