>>713058820 (OP)Because they needed to make people unrealistically evil to push their retarded sob story and their "humans are the real monsters" narrative.
No normal person would look a little girl and her father in the eyes and gun them down because some retard on the radio told them to. He even questions the order and hesitates, showing that he wasn't some sadistic psychopath. In real life he would have let them go.
They were not enemy combatant or people from an enemy nation. He had no reason to follow the order especially since his CO would never have known if he spared them.