Mahabharata epic probably has an underlying philosophical meaning. e.g. hundred Kauravas(bad princes) and five pandavas(good princes) hints at the facts that in a human being faces very few virtues (represented by pandavas) and hundreds of vices (like kauravas) all the time. It is left to us to lead our life as per the values portrayed in Mahabharata, conquer the evil vices and embrace the virtues. So Pandavas defeating kauravas is a representation of victory of good over evil. To stress this, the names of kauravas are also symbolic.
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As you pointed out the word du though means bad it must be understood as tough and terrifying in the battle. for a kshatriya and a conqueror that is a good quality. and duhsasana ( being the prince's brother) was named as he was to implement the emperor's command ( as the blind always wished that his eldest son must the next emperor) in a strict manner. here duh must be understood as strict. saasana means command/law etc and hence one who implements the law strictly.