In British slang, "nonce" is a highly offensive term for a sex offender, particularly one who has harmed children. It is considered derogatory and should be used with caution.

I'm guessing this is very context/region dependant. Calling someone a nonce as a bit of banter would be more acceptable than calling them a paedophile when I was growing up. I assume because using the officially recognised term made your accusations seem more ernest, though I've never actually thought much about it before.