Shouldn't there be の to signify that 悪 modifies 魔? It seems to be a one word
and not a phrase, how do we know if a double kanji is such a single word and
not two separate ones when reading a text?
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2016-02-18T23:54:38.557
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2016-02-19T03:36:31.743
2016-02-19T03:36:31.743
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question
particle-の compounds modification
Why is it 悪魔 and not 悪の魔?
> Shouldn't there be の to signify that 悪 modifies 魔?

No. This term was borrowed in its entirety from Chinese, as a single term.

Borrowing aside, two-character compounds are generally parsed as single terms
-- even those coined in Japan.

> It seems to be a one word and not a phrase, how do we know if a double kanji
> is such a single word and not two separate ones when reading a text?

Fundamentally, one learns to recognize such things over the course of time in
studying the language.

To better answer your question, as above, it's a good bet that many (most?)
kanji pairs that don't have kana in between are probably compounds. Many
(most?) such compounds are also read using the _on'yomi_ , which is useful
information when trying to either pronounce these terms or look them up in a
dictionary.
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2016-02-19T02:39:51.500
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2016-02-19T02:39:51.500
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answer