Some interesting insults:

Your existence is a derogatory remark against all remotely-sentient life.

The ability to conceive your inexistent humor is best reserved for the most ineffectual, toxic cancers of consciousness.

To have spoken to you has been a curse upon my bloodline in both past and future.

Inhaling in zealous, flesh-eviserating fire ants would have been more pleasant than being caught in the same room as you.

Your mockery of human-like resemblance had stuck God cold and the left the devil in perpetual convulsions of vomiting and seizure.