Sankhaya Yog-Bhagavad Gita Chapter 2 [Contents of the Gita Summarized]
Arjun Said > Shaloka: 5
It is better to live in this world by begging than to live at the cost of the lives of great souls who are my teachers. Even though they are avaricious, they are nonetheless superiors. If they are killed, our spoils will be tainted with blood.
According to scriptural codes, a teacher who engages in an abominable action and has lost his sense of discrimination is fit to be abandoned. Bhisma and Drona were obliged to take the side of Duryodhana because of his financial assistance, although they should not have accepted such a position simply on financial considerations. Under the circumstances, they have lost the respectability of teachers. But Arjuna thinks that nevertheless they remain his superiors, and therefore to enjoy material profits after killing them would mean to enjoy spoils tainted with blood.