i Register
In some senses, rancor is marked as archaic, US, rare. Watch for register when choosing this word.
noun
A feeling of long-lasting ire for another, sometimes to the point of hatred, over a perceived wrongdoing; bitterness.
I could almost see the rancor in his eyes when he challenged me to a fight.
Rancidity, rankness.
verb
To rankle or fester.