Every action is covered by some kind of fault in this world. Arjuna asks that because the activities of ksatriya are in the mode of passion, so is it not better for him to become a brahmana for it will be pure?

Krishna says, even fire is covered by smoke so by changing the varna one will not be able to overcome the faults. Even the actions of Brahmanas have faults but one shouldn't consider those faults while doing their prescribed duties according to one's nature. It is better to perform one's own duty imperfectly than to perform another's duty perfectly because one will not incur sin by performing the prescribed duties of one's own nature even when it is faulty.

If Arjuna were to act with proper consciousness while killing in the battlefield for establishing righteousness according to religious principles, he would never incur sin. If one works in proper understanding that everything is carried out only after the sanction of the Supreme will and sees the self as instruments then one will not incur sin even if he kills the whole universe what to speak of Bhisma or Drona?

One will not incur sin even if he kills the whole universe when situated in proper consciousness