naino rajñah praja-bhartur
dharma-yuddhe vadho dvisam
iti me na tu bodhaya
kalpate sasanam vacah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There is no sin for a king who kills for the right cause, who is engaged in maintaining his citizens. But this injunction is not applicable to me.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Yudhisthira thought that although he was not actually involved in the administration of the kingdom, which was being carried on well by Duryodhana without harm to the citizens, he caused the killing of so many living beings only for his personal gain of the kingdom from the hands of Duryodhana. The killing was committed not in the course of administration but for the sake of self-aggrandizement, and as such he thought himself responsible for all the sins.