Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 

 

When getting married, the ksatriya kings would take on several other youthful girls along with the married princess. These girl attendants of the king were known as dasis, or attendant mistresses. By intimate association with the king, the dasis would get sons. Such sons were called dasi-putras. They had no claim to a royal position, but they would get maintenance and other facilities just like princes. Vidura was the son of such a dasi, and he was thus not counted amongst the ksatriyas. King Dhrtarastra was very affectionate toward his younger dasi-putra brother, Vidura, and Vidura was a great friend and philosophical advisor to Dhrtarastra. Duryodhana knew very well that Vidura was a great soul and well-wisher, but unfortunately he used strong words to hurt his innocent uncle. Duryodhana not only attacked Vidura’s birth, but also called him an infidel because he seemed to support the cause of Yudhisthira, whom Duryodhana considered his enemy. He desired that Vidura be immediately put out of the palace and deprived of all his possessions. If possible, he would have liked him caned until he was left with nothing but his breath. He charged that Vidura was a spy of the Pandavas because he advised King Dhrtarastra in their favor. Such is the situation of palace life and the intricacies of diplomacy that even a faultless person like Vidura could be charged with something abominable and punished. Vidura was struck with wonder at such unexpected behavior from his nephew Duryodhana, and before anything actually happened, he decided to leave the palace for good.