Question
Is it stated explicitly anywhere in the Mahabharata that a progressively adharmic War made Ashwatthama a monster?
Answer
Only person who thought in the lines of Drona being killed with fraud caused change to Aswattama is Dhritarashtra while speaking with Sanjaya but it is not explicitly stated anywhere else.
We can infer it from the following words
"Dhritarashtra said, 'After Drona had been slain with the aid of fraud, and the Narayana weapon baffled, what, indeed, did Drona's son, thus urged by Duryodhana then, do, beholding the Parthas once more arrived for battle freed from the Narayana weapon, and careering at the head of their divisions?'
The pent up frustration at the perceived injustice and inability to bind the pandavas with his most powerful weapon might have taken Aswattama down the path mentioned in the question which is natural path he would take based on his inherent qualities that are not supposed to be there for a honour bound warrior.
Source:http://www.sacred-texts.com/hin/m07/index.htm
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