rancorous

showing deep-seated resentment

Full of rancor; evincing, or caused by, rancor; deeply malignant; implacably spiteful or malicious; intensely virulent.

Latin rancor, meaning bitter

“He is not eminently, that is to say, not saliently, selfish; not rancorous, not obtrusive--tata-ta-ta.”

— George Meredith, Complete Project Gutenberg Works of George Meredith

“On the second day of the convention, there was an intense debate about the ethics of Mars terraforming, which science writer Oliver Morton described as 'rancorous'.”