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

acerb acerbate acerbic acid acidic acidulent acidulous acrid acrimonious antagonistic antipathetic avenging belligerent bitter burning caustic choleric clashing colliding conflicting despiteful embittered envenomed evil full of hate grudgeful hateful hostile implacable irreconcilable malevolent malicious malign malignant punitive punitory quarrelsome rankled repugnant resentful resenting retaliatory revanchist revengeful set against sore spiteful splenetic stewing unappeasable venenate vengeful venomous vicious vindicatory vindictive virulent vitriolic wicked