Complex sentence: When Lee about to finish, a stranger then came out and watched Lee and warned him "IF YOU COME BACK I'LL KILL YOU!!" alerting Christa and the group.
Split into simpler sentences: When Lee about to finish, a stranger then came out and watched Lee and alerting him. Lee warned him, "IF YOU COME BACK I'LL KILL YOU!!" alerting Christa and the group.