When, if ever, is it justified to specifically target civilians? Why?

The Geneva Convention lays down that civilians are not to be subject to attack ... military personnel clearly identified as having specifically non-combatant roles such ... For example if an army base in the middle of a city is bombed and a few civilians living

The general rule is that only those people fighting you are legitimate targets of attack. Those who are not fighting should not be attacked as this would violate their human rights. The Geneva Convention lays down that civilians are not to be subject to attack.