Question
How exactly did the killing of Abhimanyu violate the agreed-upon rules of war?
Answer
Abhimanyu's killing was totally dharmic.
Indian TV depicts his killing in a grossly untruthful manner But the truth from KMG's translation shows a largely Dharmic way Abhimanyu was made to pay for his youthful hubris:
http://www.sacred-texts.com/hin/m07/m07046.htm
We can discuss the many-on-one early phases of the combat - including cutting away the chariot wheel - but
the final combat is a fair one-one-one combat with equal weapons
It is possible that Abhimanyu was more fatigued than Dusshasana's son - but AFAIK there is no warrior code that you have to let your opponent rest between rounds. If he doesn't surrender and has weapon in hand or is reaching for one , he is fair game.
He went in of his own choice, could have surrendered any time (IIRC, one Kaurava warrior does say "let us take him prisoner") - and the Kauravas killed him fairly, for their self-preservation, given how devastating he had been.
With the end of his locks waving in the air, with that supreme weapon upraised in his hands, his body became incapable of being looked at by the very gods. The kings beholding it and the wheel in his hands, became filled with anxiety, and cut that off in a hundred fragments. Then that great car-warrior, the son of Arjuna, took up a mighty mace. Deprived by them of his bow and car and sword, and divested also of his wheel by his foes, the mighty-armed Abhimanyu (mace in hand) rushed against Aswatthaman. Beholding that mace upraised, which looked like the blazing thunderbolt, Aswatthaman, that tiger among men, rapidly alighted from his car and took three (long) leaps (for avoiding Abhimanyu).
He makes Ashwatthama run away
Slaying Aswatthaman's steeds and two Parshni charioteers with that mace of his, Subhadra's son, pierced all over with arrows, looked like a porcupine. Then that hero pressed Suvala's son, Kalikeya, down into the earth, and stew seven and seventy Gandhara followers of the latter. Next, he slew ten car-warriors of the Brahma-Vasatiya race, and then ten huge elephants. Proceeding next towards the car of Duhsasana's son, he crushed the latter's car and steeds, pressing them down into the earth.
He Is still powerful and dangerous.
The invincible son of Duhsasan, then, O sire, taking up his mace, rushed at Abhimanyu. saying, 'Wait, Wait!' Then those cousins, those two heroes, with upraised maces, began to strike each other, desirous of achieving each other's death, like three-eyed (Mahadeva) and (the Asura) Andhaka in the days of old. I ach of those chastisers of foes, struck with the other's mace-ends fell down on the earth, like two uprooted standards erected to the honour of Indra. Then Duhsasana's son, that enhancer of the fame of the Kurus, rising up first, struck Abhimanyu with the mace on the crown of his head, as the latter, was on the point of rising. Stupefied with the violence of that stroke as also with the fatigue he had undergone, that slayer of hostile hosts, viz., the son of Subhadra, fell on the earth, deprived of his senses. Thus, O king, was one slain by many in battle,--one who had ground the whole army, like an elephant grinding lotus-stalks in a lake.
The text doesn’t say if Abhimanyu had his weapon or not when he was killed. Duhsasana’s son was faster to rise and in the heat of the moment, he might have committed a minor infraction, in case Abhimanyu had not yet picked up his weapon. Nothing in the text suggests he bludgeoned a supine, weaponless semi-conscious opponent to death, or that Abhimanyu was not in the process of resuming the combat after picking up his weapon. And as far as I know, the endless repetition of the "adharmic killing of Abhimanyu" doesn't mention that Dusshasana's son didn't wait for Abhimanyu to pick up his weapon before hitting him.
Sanjaya shows whose side he was on but also says
one who had ground the whole army, like an elephant grinding lotus-stalks in a lake.
As Shakespeare would say - the Abhimanyu partisan doth protest too much, methinks.
Note: “The question: How exactly did the killing of Abhimanyu violate the agreed-upon rules of war?” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.