Mbh 7.197.4c

While engaged in fight he was incapable of being slain by Indra himself. The preceptor was venerable in years and always devoted to our welfare. Yet unrighteous that we are, and stained with a levity of behaviour, we scrupled not to injure him. Alas, exceedingly cruel and very heinous has been the sin that we have committed, for, moved by the desire of enjoying the pleasures of sovereignty, we have slain that Drona. My preceptor had all along been under the impression that in consequence of my love for him, I could, (for his sake) abandon all,--sire, brother, children, wife and life itself. And yet moved by the desire of sovereignty, I interfered not when he was about to be slain. For this fault, O king, I have, O lord, already sunk into hell, overcome with shame. Having, for the sake of kingdom, caused the slaughter of one who was a Brahmana, who was venerable in years, who was my preceptor, who had laid aside his weapons, and who was then devoted, like a great ascetic, to Yoga, death has become preferable to me to life!”’”