"The Sniper" by Liam O'Flaherty
Why is war terrible, especially a civil war? What is the price of being at conflict? Why are some people willing to pay that price, and even fight against their own people, morals and family?
I want to answer the last part of your question first. "Why are some people willing to fight against their own family?" With reference to the story "The Sniper," neither sniper knew it was his family member that he was shooting at. The protagonist didn't realize that he had shot and killed his brother until he went down to examine the body.
Then the sniper turned over the dead body and looked into his brother´s face.
As for why a civil war is especially terrible, it's terrible because it potentially pits family member against family member. So it not only tears a country apart, but it also tears family members apart. In light of the story, it for sure tore the family apart. Two brothers were fighting on opposite sides of the conflict. What's worse is that the family tear will never be healed, because one of the brothers is dead.
As for why anybody would participate in such a horrible thing as war, it comes down to their beliefs. When a person believes so deeply in a cause, they are willing and able to do all kinds of amazing and horrible things in pursuit of that cause.