ill will vs bitterness

ill will and bitterness both are nouns.

Word	Noun	Adjective	Verb	Adverb
ill will	Yes	No	No	No
bitterness	Yes	No	No	No

As nouns, bitterness is a hyponym of ill will; that is, bitterness is a word with a more specific, narrower meaning than ill will:

ill will (noun)	bitterness (noun)
a hostile (very unfriendly) disposition	the property of having a harsh unpleasant taste
the feeling of a hostile person	a rough and bitter manner
	the taste experience when quinine or coffee is taken into the mouth
	a feeling of deep and bitter anger and ill-will