He was more monster than man.
Why is it that no articles have been used here?
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Both monster and man are used in an adjectival way here. The fact that they are compared (more X than Y) means that the writer intends to show "being a monster" and "being a man" not as an absolute thing, but as a property a person can have more or less. In that way, it can help to think of them as nouns that behave like an adjective.
Grammatically, the sentence is comparable to:
The flag was more orange than yellow.
This construction works when you are comparing (with more or less) for nouns that are normally not adjectives. When you are not comparing, it doesn't work; you cannot say "he was man" or "he was monster".