The word rancor is best when you're not just talking about anger, you're talking about a deep, twisted bitter type of anger in your heart. The open rancor in political discussion prevents cooperation between political parties.
The most helpful way to remember rancor with all its dark, miserable bitterness is to think of how rancor rhymes with canker, as in canker sore, the horrible painful burning on your lip. Or, you might want to remind yourself that rancor has its roots in the word rancid meaning "rotten." Rancor refers particularly to the sort of ill-will associated with resentment, envy, slow-brewing anger, and a very personal sort of hatred.