Characterized by intense animosity or virulence of feeling or action; virulent.

That has become a fester (fester, n. 1a); that has festered (in various senses of the verb).

Displaying, expressing, or implying displeasure or discontent; peevish, cross. Of looks, etc.

Of a feeling or emotion: characterized by, or of the nature of, rancour.

festered, inveterate.

Full of bitterness; spiteful, malicious.

Resentful, bitter, irascible; stubborn, obstinate.

Embittered.

Flowing with bile; (figurative) bitter, rancorous.

figurative. Of language, persons, etc.: Extremely sharp, caustic, or scathing; bitterly ill-natured or malignant.