Myself and Nalakuvara prabhu were talking about the difference between dovetailing once propensities, fulfilling ones propensities, and acting with a selfish desire.
The Kauravas were Ksatriya's, so their propensity was to fight, and by fighting in the war they were fulfilling their propensities and acting according to their duties. However, they were doing so with a selfish desire, and therefore their activities could not be considered spiritual as such. Often devotees do this, they claim to be acting according to their propensities and duties, but because they are doing so without the recommendation of a spiritual master, and not for the pleasure of guru and Krsna, their actions are simply material, providing no spiritual advancement or bliss, and often create a distance between themselves and other devotees, as was the case with the Kauravas.
Then you have Arjuna, who's propensity was also to fight, and Krsna wanted him to fight, yet his own desire was to take up the life of sannyasa, claiming that to be more spiritual. This is also often carried out by devotees, who think that a certain service or position is more desirable then the one they actually have the propensity to fulfill, and thus they act on their own desire, not for Krsna's pleasure, and end up achieving neither spiritual nor material success in their endeavour, and end up frustrated.
But ultimately Arjuna sets the best example in the end, by approaching Krsna as a disciple, asking Him what the best thing is to do. Sometimes we get confused about what is actually our propensity, and what is just a whimsical desire, and even more so, what can we do to actually please Guru and Krsna. And it is actually only through the advice of the spiritual master that one can be given the proper guidance by which to act properly and make real spiritual advancement, not degrading oneself.
(see BG 4:34-35, 6:23 plus purports as reference to some of the points)