Bhagavad Gita 1-39

kulakShayE praNashyanti kuladharmaaHa sanatanaaHa

dharma naShTE kulaM kRutsnamadharmObhibhavatyuta || 39 ||

With the extinction of the family Sanatana dhrama and the family values will be destroyed. When Dharma is lost the entire family will be in the clutches of Adharma.

This great war at Kurukshetra was considered to be Dharma Yuddha. It was not just for the kingdom. It was a war of Dhrama versus Adharma; good versus evil. The purpose of the war was to protect the good so that it prevails and is reestablished. Arjuna questions whether this great war would achieve that. When majority in the family, clan or dynasty are killed who will be remaining to follow the right values? It will result in a catastrophic situation where there will be nobody to guide or follow the Dharma. Then only Adharma is going to prevail and rule. Is this what we want? This war does not seem the serve the very purpose for which it is fought. Arjuna rightly questions why he should then fight this battle when it does not serve the purpose for which it is fought.