Born of a sage, trained for war
Droṇācārya is the son of the sage Bharadvāja, and his early life is marked by poverty despite his brahminical birth. A childhood friendship with Drupada, then a prince and later a king, ends badly: Drupada, once he ascends the throne, refuses to acknowledge the earlier bond of equality between them — a humiliation that sets in motion much of Droṇa's later story, since the eventual instrument of his revenge against Drupada is the very generation of princes he is about to train.
Teacher to both sides
Droṇa becomes the preceptor of both the Kaurava and Pāṇḍava princes at Hastināpura, training them together in dhanurveda — the science of archery and martial arts — without, in principle, favouring either branch of the family. Arjuna emerges as his most gifted and most favoured student, a preference the text does not hide, and one that has consequences for how the rest of the epic's central rivalry plays out.
The Ekalavya parable
The most morally uncomfortable episode associated with Droṇa is his encounter with Ekalavya, a low-caste hunter's son who, refused formal instruction, teaches himself archery by practising before a clay image of Droṇa and surpasses even Arjuna in skill. When Droṇa discovers this, he demands Ekalavya's right thumb as guru-dakṣiṇā (a teacher's due), crippling his archery permanently and preserving Arjuna's claim to be unmatched. The episode is treated by the epic itself, and by centuries of subsequent commentary, as a genuine ethical fault line — a story about the costs of a teacher's loyalty to caste and to a favoured student, not a straightforwardly heroic anecdote.
Commander-in-chief, and a death by deception
During the Kurukṣetra war, Droṇa serves as commander-in-chief of the Kaurava army for four days following Bhīṣma's fall, and proves militarily formidable enough that the Pāṇḍavas resort to a calculated deception to end his resistance: Yudhiṣṭhira allows Droṇa to believe, through a deliberately ambiguous statement, that his son Aśvatthāmā has been killed. Droṇa lays down his weapons in grief and is killed while unarmed — an ending the epic frames, like much of the war's final stretch, as morally compromised on every side rather than as a clean victory for either.