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Ashwatthama is considered a partial avatara (amsha) of Shiva..
When Ashwatthama is still a child, Dronacharya is insulted by King Drupada (father of Draupadi) when he goes to ask for a cow to feed milk to his son. It is only after this incident that Kripacharya offers him a position as a teacher to the Pandavas and Kauravas. There perhaps was some anger in Ashwatthama towards Drupada (and hence Draupadi) due to this slight.
Ashwatthama's real enmity towards the Pandavas starts when they kill Dronacharya on the battlefield due to a 'lie' by Yudhishtira. When Drona is told by many that Ashwatthama is dead (not entirely untrue, because an elephant by the same name had died), he refuses to believe anybody but Yudhishtira. Yudhishtira confirms the death of the elephant, but Krishna blows his conch to blot out Yudhisthira's reference to the elephant. Drona, on hearing this, drops his weapons, and sits down in meditation on the battlefield. Seizing the opportunity, the Pandavas (Arjuna?) finish him off. Surely this must have angered Ashwatthama enough to commit all the atrocities he eventually does at the end of the war.
As a side note, it is interesting that there are several warriors in the Mahabharata that are considered unbeatable at one point or another. Bhishma is the first, whom even Parashurama is unable to beat (the Amba story), and he is only taken out when Shikhandhi is used to make him drop his weapons. The second is Arjuna, although his case is slightly weak, given that he is protected against Karna once or twice. Drona is deemed unbeatable in the Mahabharata war (although he is beaten by Arjuna in the Virata parva battle), and hence the Pandavas resort to trickery to take him out. Bhimasena is of course invincible and he routs most of the key bad characters in the entire Mahabharata.
Towards the end of the war, Ashwatthama resorts to killing several warriors in the Pandava camp including the sons of the Draupadi/Pandavas (mistakenly assuming that they were the Pandavas themselves) in stealth at night, not becoming of even a lowly soldier let alone a top warrior in an army.
Ashwatthama also directs his brahmAstra at the womb of Uttara (daughter of king Virata, wife of Abhimanyu) to finish off the as-yet unborn Parikshit, since he would be the last of the Kuru clan. Krishna uses his chakra to protect the child and mother, but only ends up reviving the child after a still-birth.
Lastly, Duryodhana, in his death bed, requests Ashwatthama to have a child with his wife Banumati, so that that child can be the ruler of Hastinapura. While Ashwatthama does not actually oblige, he neither accepts not refuses to Duryodhana, not a good quality for someone who should immediately know the right Dharma.
On the last item, Vyasa himself sires Pandu and Dhritarashtra (with Ambika and Ambalika) to keep the Kuru clan alive. Is that considered Dharma because he is a rishi and God Himself? Or is it because Vyasa is a son of Satyavati (the fisherman's daughter who is married to Shantanu) and he is obeying his mother's command and acting on behalf of his now-dead younger brother Vichitravirya? In this case, the continuation of the clan is for the good of humanity, perhaps, while not so in the case of Duryodhana?