Bg 2.5

gurun ahatva hi mahanubhavan
sreyo bhoktum bhaiksyam apiha loke
hatvartha-kamams tu gurun ihaiva
bhunjiya bhogan rudhira-pradigdhan

Word for word: 
gurun — the superiors; ahatva — not killing; hi — certainly; maha-anubhavan — great souls; sreyah — it is better; bhoktum — to enjoy life; bhaiksyam — by begging; api — even; iha — in this life; loke — in this world; hatva — killing; artha — gain; kaman — desiring; tu — but; gurun — superiors; iha — in this world; eva — certainly; bhunjiya — one has to enjoy; bhogan — enjoyable things; rudhira — blood; pradigdhan — tainted with.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It would be 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 desiring worldly gain, they are superiors. If they are killed, everything we enjoy will be tainted with blood.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.