Which term best describes feelings or attitudes marked by deep-seated ill will?

The term that best describes feelings or attitudes marked by deep-seated ill will is "invective." This word refers to abusive or vitriolic language used to criticize or express hostility toward someone or something. It captures the essence of strong negative emotions, suggesting not just a passing dislike but a deeper, more enduring animosity that can be communicated through harsh words and vituperative statements. This makes it particularly relevant in contexts where intense disdain is being expressed.

In contrast, the other terms present different connotations. "Amity" refers to a friendly and peaceful relationship, which is the opposite of ill will. "Agape" denotes an unconditional love, typically used in a spiritual or philosophical sense, further underscoring a sentiment diametrically opposed to hostility. "Languorous," on the other hand, describes a state of weakness or fatigue, often with a sense of dreamy relaxation, and does not relate to feelings of animosity or ill will at all. Thus, "invective" stands out as the most appropriate term for describing deep-seated ill will.