The killing of Abhimanyu was purely Dharmic - but even accepting Krishna's word for it as he addressed the defeated Duryodhana -
"'Vasudeva said, "Thou, O son of Gandhari, hast been slain with thy brothers, sons, kinsmen, friends, and followers, only in consequence of the sinful path in which thou hast trod! Through thy evil acts those two heroes, Bhishma and Drona, have been slain! Karna too hath been slain for having imitated thy behaviour! Solicited by me, O fool, thou didst not, from avarice, give the Pandavas their paternal share, acting according to the counsels of Shakuni! Thou gavest poison to Bhimasena! Thou hadst, also, O thou of wicked understanding, endeavoured to burn all the Pandavas with their mother at the palace of lac! On the occasion also of the gambling, thou hadst persecuted the daughter of Yajnasena, while in her season, in the midst of the assembly! Shameless as thou art, even then thou becamest worthy of being slain! Thou hadst, through Subala's son well-versed in dice, unfairly vanquished the virtuous Yudhishthira who was unskilled in gambling! For that art thou slain! Through the sinful Jayadratha again, Krishna was on another occasion persecuted when the Pandavas, her lords, had gone out hunting towards the hermitage of Trinavindu! Causing Abhimanyu, who was a child and alone, to be surrounded by many, thou didst slay that hero. It is in consequence of that fault, O sinful wretch, that thou art slain! All those unrighteous acts that thou sayest have been perpetrated by us, have in reality been perpetrated by thee in consequence of thy sinful nature! Thou didst never listen to the counsels of Brihaspati and Usanas! Thou didst never wait upon the old! Thou didst never hear beneficial words! Enslaved by ungovernable covetousness and thirst of gain, thou didst perpetrate many unrighteous acts! Bear now the consequences of those acts of thine!"
The only adharmic act in the battlefield charged by Krishna was the killing of Abhimanyu - whereas he himself admitted the Adharmic killing of just about ALL the mighty warriors on the Kaurava side - Bhishma,Drona (Satyaki was ready to kill drishtadyumna for it ), Karna, Bhurishravas and Duryodhana.
"Sanjaya continued, 'Upon the conclusion of these words of the intelligent king of the Kurus, a thick shower of fragrant flowers fell from the sky. The Gandharvas played upon many charming musical instruments. The Apsaras in a chorus sang the glory of king Duryodhana. The Siddhas uttered loud sound to the effect, "Praise be to king Duryodhana!" Fragrant and delicious breezes mildly blew on every side. All the quarters became clear and the firmament looked blue as the lapis lazuli. Beholding these exceedingly wonderful things and this worship offered to Duryodhana, the Pandavas headed by Vasudeva became ashamed. Hearing (invisible beings cry out) that Bhishma and Drona and Karna and Bhurishrava were slain unrighteously, they became afflicted with grief and wept in sorrow. Beholding the Pandavas filled with anxiety and grief, Krishna addressed them in a voice deep as that of the clouds or the drum, saying, "All of them were great car-warriors and exceedingly quick in the use of weapons! If ye had put forth all your prowess, even then ye could never have slain them in battle by fighting fairly! King Duryodhana also could never be slain in a fair encounter! The same is the case with all those mighty car-warriors headed by Bhishma! From desire of doing good to you, I repeatedly applied my powers of illusion and caused them to be slain by diverse means in battle. If I had not adopted such deceitful ways in battle, victory would never have been yours, nor kingdom, nor wealth! Those four were very high-souled warriors and regarded as Atirathas in the world. The very Regents of the Earth could not slay them in fair fight! Similarly, the son of Dhritarashtra, though fatigued when armed with the mace, could not be slain in fair fight by Yama himself armed with his bludgeon! You should not take it to heart that this foe of yours hath been slain deceitfully. When the number of one's foes becomes great, then destruction should be effected by contrivances and means. The gods themselves, in slaying the Asuras, have trod the same way. That way, therefore, that hath been trod by the gods, may be trod by all.
And if I am not mistaken, the FIRST Adharmic battlefield act was initiated by the Pandavas - the incapacitation of Bhishma.
Ashwatthama's night slaughter was done by Siva who had entered his body. He killed them because their time had come. So it can be taken off the ledger between Kauravas and Pandavas.
Who committed more acts of Adharma in the war - The Pandavas or the Kauravas?