Complex sentence: Mag is shot in the legs by a Butcher While tending to her, Ballard is killed by a stray shot to the head, leaving Zone to carry Mag through the tunnels.
Split into simpler sentences: Mag is shot in the legs by a Butcher and Ballard goes back to tend to her. However, he is killed by a stray shot to the head, leaving Zone to carry Mag through the tunnels.