What constitutes a civilian during wartime? If a civilian is, according to the simple definition, a member of the citizen body not actively engaged in combat, then killing civilians can certainly be justified. A civilian can be outside a combat zone, yet still house enemy combatants or provide information to the enemy military.
The intentional killing of civilians is not justified by the rules of war. However the unintentional killing of civilians does happen often and is usually excused all nations as an unfortunate consequence of the nature of war. Thus it is really a question of whether the killing was intentional or not.