Derogatory: detracting from the character or standing of something/someone. disparaging or slanderous remarks. — Merriam-Webster
synonyms: defamatory, libelous, slanderous
Ad hominem: Latin for "to the man" or "to the person", short for argumentum ad hominem, it is a type of logical fallacy, where during an argument or debate, you start making personal attacks directed against a person or that person's character, rather than towards the object or subject of debate or the issues at hand.
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Polemics: the art or practice of engaging in controversial debate or dispute.
— Oxford Living Dictionaries
Also, possibly: Abusive language, Mischaracterization (of events or a person).
And, here are a bunch of other, more esoteric words that you probably didn't mean:
Denigration: The action of unfairly criticizing someone or something. — Oxford Living Dictionaries
Vituperation: Bitter and abusive language. — The Oxford English Mini Dictionary, Eighth Edition
Scurrilous: Making or spreading scandalous claims about someone with the intention of damaging their reputation. — Oxford Living Dictionaries
Traduce: Speak badly of or tell lies about (someone) so as to damage their reputation
— The Oxford Dictionary of Difficult Words, 2004
Enmity: The state or feeling of being actively opposed or hostile to someone or something.
— Oxford Living Dictionaries
Invective: Insulting, abusive, or highly critical language. —The Oxford Dictionary of English, 2010
Lambaste: Criticize (someone or something) harshly — The Oxford Dictionary of English, 2010
Harangue: A lengthy and aggressive speech or lecture — The Oxford Dictionary of English, 2010
Vilification: To vilify, to make someone the villain, abusively disparaging speech or writing.
— The Oxford Dictionary of English, 2010
Antipathy: A deep-seated feeling of dislike. — The Oxford Dictionary of English, 2010