Ashwatthama is reporting his night-slaughter to the dying Duryodhana:
Going from this world, O monarch, and meeting with those mighty car-warriors (that have preceded thee), show thy regards to them, at my request, one after another, according to the order of their rank and years. Having offered worship to thy preceptor, that foremost of all wielders of bows, tell him, O king, that Dhrishtadyumna hath been slain by me. Embrace king Bahlika, that mighty car-warrior, as also the ruler of the Sindhus, and Somadatta, Bhurishrava, and the other foremost of kings that have preceded thee to heaven. At my request, embrace all of them and enquire after their welfare."
Sanjaya continued, "Having said these words unto the king deprived of his senses and lying with broken thighs, Ashvatthama once more cast his eyes on him and uttered these words, "If, O Duryodhana, thou hast any life in thee still, listen to these words that are so pleasant to hear. On the side of the Pandavas, only seven are alive, and among the Dhartarashtras, only we three! The seven on their side are the five brothers and Vasudeva and Satyaki; on our side, we three are myself and Kripa and Kritavarma! All the sons of Draupadi have been slain, as also all the children of Dhrishtadyumna! All the Pancalas too have been slain, as also the remnant of the Matsyas, O Bharata! Behold the vengeance taken for what they had done! The Pandavas are now childless! While buried in sleep, the men and animals in their camp have all been slain! Penetrating into their camp in the night, O king, I have slain Dhrishtadyumna, that wight of sinful deeds, as one kills an animal."
Duryodhana then, having heard those words that were so agreeable to his heart, regained his senses and said these words in reply, "That which neither Ganga's son, nor Karna, nor thy sire, could achieve, hath at last been achieved by thee today, accompanied by Kripa and Bhoja. Thou hast slain that low wretch (Dhrishtadyumna) who was commander of the Pandava forces, as also Shikhandi. In consequence of this I regard myself equal to Maghavat himself! Good be to you all! Let prosperity be yours! All of us will again meet together in heaven!"
Having said these words the high-souled king of the Kurus became silent. Casting off his griefs for all his (slain) kinsmen, he then gave up his life-breath. His soul ascended to sacred heaven,
It cannot be argued that he went to Veera-swarga, since his army allegedly committed war crimes.
Does the Mahabharata say why he went to heaven?