Directly negative:
* Abuse: stronger than insult, often involving harsh language.
* Slur: a derogatory remark, especially based on race, religion, etc.
* Contempt: showing a feeling of disdain or scorn.
* Disparagement: to speak poorly of someone or something.
* Derision: mocking or ridiculing someone.
* Vituperation: harsh and abusive language.
More subtle or indirect:
* Slight: a small offense or disrespect.
* Affront: an open insult or act of disrespect.
* Offense: something that causes anger or displeasure.
* Indignity: a humiliating or disrespectful treatment.
* Rebuff: to rudely reject or refuse something.
More formal or literary:
* Calumny: false and malicious statements intended to damage someone's reputation.
* Aspersion: a damaging remark or accusation.
The best synonym for "insult" will depend on the specific context and the level of negativity you want to convey.