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

Tamar couldn't believe her old colleague, Zane, was running the coffee shop across from hers in Johannesburg. Their rancorous texts escalated until Tamar realized she was arguing over a mere caffeine rivalry. She laughed, and so did Zane, when they met for a cup of peace.

At the edge of Helsinki's frozen Herttoniemi Beach, Tariq faced his old rival, ice shards crunching underfoot with each tense step. With a glare that could freeze summer air, he hurled an icicle-shaped snowball that shattered against the other's coat, the rancorous gesture echoing the bitter feud they'd nurtured since childhood.

“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'.”