He swore revenge out of years of deep-seated resentment.
There is deep enmity between the two.
怨恨 is a stiff, legal/journalistic term for a grudge (often as a motive for crime), while 恨み is the common everyday word.
Compound of 怨 (grudge, resentment) and 恨 (regret, resentment). The exact historical derivation is uncertain, but the combination intensifies the sense of deep-seated ill will.