Bhagavad Gita 2.5गुरूनहत्वा हि महानुभावान् श्रेयो भोक्तुं भैक्ष्यमपीह लोके।हत्वार्थकामांस्तु गुरूनिहैव भुञ्जीय भोगान् रुधिरप्रदिग्धान् ॥

According to scriptural codes, a teacher who engages in an abominable action and has lost his sense of discrimination is fit to be abandoned. Bhīṣma and Droṇa 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.