The question has already been answered.
However a portion of the description, in the question, placed as under, needs to be put in the proper perspective, for that is not the way, how, Dronacharya put his weapons down{or collapsed on the warfield}. Especially so,for those who may not be aware of the details of the event,as given in the Mahabharata epic.
“And since Ashwathama Hataha means Ashwathama is dead, assuming his son to be dead in the great war, Drona collapses and seeing this as a cue, Draupadi's brother Drushtadyumna beheads Drona”.
{1}The illustrious Dronacharya, did not collapse,immediately on hearing about the death of his son Ashwatthaman. So, what did the illustrious Dronacharya do subsequently? This is explained as under
Recollecting however, the prowess of his son, he soon came to regard that intelligence as false. Hearing, therefore, of his slaughter, Drona did not become unmanned. Indeed, soon recovering his senses, he became comforted, remembering that his son was incapable of being resisted by foes.
{2}Dronacharya’s also subsequently invoked into existence, God Brahma’s weapon, {after hearing Ashwatthaman was dead}.
Filled with wrath and desirous of compassing the destruction of those brave Panchalas, that mighty car-warrior, that scorcher of foes, viz., Drona, dispelling all those shafts of the Panchalas, then invoked into existence the Brahma weapon
In consequence of fallen elephants and steeds, O Bharata, the earth, miry with flesh and blood, became impassable. Having slain twenty thousand Panchala car-warriors, Drona, in that battle, shone resplendent like a smokeless, blazing fire. Once more filled with rage, the valiant son of Bharadwaja cut off, with a broad-headed arrow, the head of Vasudana from his trunk. Once more slaying five hundred Matsyas, and six thousand elephants, he slew ten thousand steeds.
{3}The Saptarishis and other distinguished Rishis, then addressed Dronacharya
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.
{4}What did the Saptarishis and others tell Dronacharya
Thou art fighting unrighteously. The hour of thy death is come. Laying aside thy weapons in battle, O Drona, behold us stationed here. After this, it behoveth thee not to perpetrate such exceedingly cruel deeds. Thou art versed in the Vedas and their branches. Thou art devoted to the duties enjoined by truth, especially, thou art a Brahmana. Such acts do not become thee. Lay aside thy weapons. Drive away the film of error that shrouds thee. Adhere now to the eternal path. The period for which thou art to dwell in the world of men is now full. Thou hast, 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.
{5}The words of Yudhishthira, did play a role in Dronacharya keeping his weapons down. But, that was not the “only reason” why Dronacharya, placed his weapons down. Dronacharya was filled with quilt, because he had invoked into existence the “weapon of God Brahma” and learned that he was a great offender against the high-souled Pandavas.
The words of the Saptarishis, induced this AWARENESS in Dronacharya.
Hearing those words from Yudhishthira, the mighty car-warrior Drona, afflicted with grief, for the (supposed) death of his son, yielded to the influence of despair. By the words, again, of the Rishis, he regarded himself a great offender against the high-souled Pandavas. Hearing now about the death of his son, he became perfectly cheerless and filled with anxiety; upon beholding Dhrishtadyumna, O king, that chastiser of foes, could not fight as before.'"
The above said narration in the Mahabharata epic, also suggests, that the illustrious Dronacharya, being a very pious Brahmin, would have given very high importance to the advice of the Saptarishis{as also weightage to the words of his pupil Yudhishthira}.And since the illustrious Dronacharya, was a man of great principles, he found it very difficult to continue fighting on Duryodhana's side{side of Adharma}, after the aforementioned events.
Reference:Mahabharata Book:7, Drona Parva,Drona-Vadha Parva
Reference link:- http://www.sacred-texts.com/hin/m07/m07187.htm
