Ashwatthama, son of Guru Dronacharya, was an unsung warrior, overshadowed by the greats of his time. A part Shiva avatar, he was one of the fiercest in Kaurava camp. It is said that Ashwatthama had equal liking for both Kauravas and Pandavas, but chose to fight on the Kauravas side since Dronacharya was on Kauravas side.
The Pandavas would have lost the war much earlier had Krishna not saved them multiple times. After Dronacharya was killed, Ashwatthama released his celestial weapon – Narayanastra. This weapon sent thousands of arrows each of which turned into a deadly snake on reaching the target. Krishna asked the whole Panadava army and Pandavas to surrender to this weapon, as the weapon did not hurt an unarmed warrior. However Bheema was reluctant to do that and started fighting with the astra , which was slowly overpowering him. However Arjuna and Yudhisthir ran and subdued Bheema and asked him to surrender, when the astra retreated.
That being said, Ashwatthama was death incarnated when he was angry, which happened on the concluding day of Mahabharata war, before he was cursed.
Dronacharya wanted to reserve the best of warrior skill training for Ashwatthama, until Arjuna started following them everywhere. It is only when Arjuna saved Dronacharya from a crocodile attack one day that Dronacharya decided to make Arjuna his main pupil.
So Ashwatthama was born in a time, when there were many other demigods of equal or greater strength. He was not less in power or skills than anyone else, however the whole of Mahabharata is focused on Pandavas and Kauravas, and so some other excellent warriors such as Ashwatthama, Eklavya, Balarama got little focus.