What were the mistakes committed by Droṇachārya?

 

A Vipra/brāhmaṇa’s duty is learn scriptures and teach them to students, give advice to the Kings.

Droṇa, being a Vipra/brāhmaṇa should not have deviated from his Dharma and joined the army of Kauravas, which is a Kshatriya Dharma, to fight with the army of Pandavas.

1. Drona, when he was in poverty, felt that he had been insulted by Drupada, a childhood friend of the former and King of Panchala kingdom
2. Later, he became the teacher of Kauravas and Pandavas and taught them the art of fighting using various weapons. So, he and his family members were given shelter in Hastinapur and were sufficiently paid.
3. Drona’s 1st mistake, being a Brahmana, was to nurture the hatredness towards King Drupada and asked Kauravas and Pandavas, after their education period was over, to defeat Drupada and bring him to his presence, a token of Guru Dakshina - Present made to the Guru on the completion of a course of instruction.
4. Drona’s 2nd mistake was to divide Panchala Kingdom, after Drupada was defeated, and to take half of his kingdom.
5. Drona’s 3rd mistake was to continue to remain in the Assembly of Kauravas, even after their education is completed, though he had been aware of growing hatredness of Duryodhana towards Pandavas. As he remained in the Assembly of Kauravas, he was forced to witness the humiliation meted out to Pandavas by Duryodhana and others, after game of dice.
6. When Duryodhana refused to part with the kingdom of Pandavas, after completion of 12 years’ period of exile and 1 year period of living incognito, Drona remained helpless. He should have quit that place, being a brāhmaṇa and attained the age of 85 years, and should have engaged in austerities. Still, he remained there itself and participated in the Mahabharata war.
7. Why should Drona participate in the war, when he was not involved in tussle between Kauravas and Pandavas? This was the greatest blunder committed by Drona. He, alongwith his son Asvatthama and his brother-in-law Kripa, participated in the Mahabharata war.
8. His mental calibre had so degraded by 13th day of the war that he alongwith other Kaurava warriors killed Abhimanyu, through deceit.

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On the 15th day of the war, Drona had become so overwhelmed with anger, and killed ordinary warriors and kings by discharging Brahmastra.

In order to demoralise Drona, Bhimasena killed an elephant with the name Aswatthaman, and proclaimed before Drona that he had killed Aswatthaman. Drona thought it was Aswatthaman, his son, but still he continued his battle.

Beholding Drona stationed on the field for the extermination of the Kshatriya race, the Rishis Viswamitra, and Jamadagni, and Bharadwaja, and Gautama, and Vasishtha, and Kasyapa, and Atri, and the Srikatas, the Prisnis, Garga, the Valkhilyas, the Marichis, the descendants of Bhrigu and Angiras, and diverse other sages of subtle forms quickly came thither, with the Bearer of sacrificial libations at their head, and, desirous of taking Drona unto the region of Brahman, addressed Drona, that ornament of battle, and said,

'You are fighting unrighteously. The hour of your death is come. Laying aside thy weapons in battle, O Drona, behold us stationed here. After this, it is fitting for you not to perpetrate such exceedingly cruel deeds.

You are versed in the Vedas and their branches. You art devoted to the duties enjoined by truth, especially, you are a Brahmana. Such acts do not become you. Lay aside thy weapons. Drive away the film of error that shrouds you. Adhere now to the eternal path.

The period for which you are to dwell in the world of men is now full. You have, with the Brahma weapon, burnt men on earth that are unacquainted with weapons.

This act that thou hast perpetrated, O regenerate one, is not righteous. Lay aside thy weapons in battle without delay, O Drona, do not wait longer on earth. Do not, O regenerate one, perpetrate such a sinful act.'

Bhima reminds Droṇa of his downfall from his Varṇāśrama Dharma.

tato bhīmo dṛḍhakrodho droṇasyālśiṣya taṃ ratham
śanakair iva rājendra droṇaṃ vacanam abravīt

yadi nāma na yudhyerañ śikṣitā brahma bandhavaḥ
svakarmabhir asaṃtuṣṭā na sma kṣatraṃ kṣayaṃ vrajet

ahiṃsā sarvabhūteṣu dharmaṃ jyāyastaraṃ viduḥ
tasya ca brāhmaṇo mūlaṃ bhavāṃś ca brahmavittamaḥ

śvapākavan mleccha gaṇān hatvā cānyān pṛthagvidhān
ajñānān mūḍhavad brahman putradāradhanepsayā

ekasyārthe bahūn hatvā putrasyādharmavid yathā
svakarmasthān vikarmastho na vyapatrapase katham

sa cādya patitaḥ śete pṛṣṭenāveditas tava
dharmarājena tad vākyaṃ nātiśaṅkitum arhasi

Then Bhima, of great wrath, holding the car of Drona, O monarch, slowly said these words unto him:

If wretches amongst Brahmanas, discontented with the avocations of their own order, but well-versed in arms, did not fight, the Kshatriya order then would not have been thus exterminated.

Abstention from injury to all creatures have been said to be the highest of all virtues. The Brahmana is the root of that virtue.

As regards thyself, again, you are the foremost of all persons acquainted with Brahma. Slaying all those Mlecchas and other warriors, who, however, are all engaged in the proper avocations of their order, moved thereto by ignorance and folly,

O Brahmana, and by the desire of wealth for benefiting sons and wives; indeed, for the sake of an only son, why do you not feel ashamed?

He for whom you have taken up weapons, and for whom you live, he (Aswatthama), deprived of life, lies today on the field of battle, unknown to you and behind thy back. King Yudhishthira the just had told thee this. It is fitting for you not to doubt this fact.'

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That was the story of Drona, who had fallen from the status of great teacher to an ordinary mortal.