Depends on your relationship, if you know that you can basically say "You disgust me" to the person you are talking to and they will not get offended, you can say it amicably. As if I see you pick up a candy bar from the trash and eat it: if we are friends taking a walk maybe you'll shrug it off, while if you are a random homeless person I see on the street, you'll maybe get angry at me. On its use as a synonym to "kinky", or related to sexual behavior anyway, it's another case and Dan's answer applies.

"Nasty" is not a bad word. It is perfectly good standard English. It can be used in ordinary speech. It could be used in a work situation: "We're having a nasty problem with defective chips in the Model 1640B." It could be used in everyday conversation: "I spent the weekend on the beach, and I got a nasty sunburn." It can be used to describe a person who is mean, or a bully. "Stay away from him, he's a really nasty guy." "She's usually a sweet person, but watch out for her nasty streak." However, one of the meanings of the word "nasty," (like the word "dirty,") is "immoral, indecent, sexual in a bad way." Depending on the context, the phrase "a nasty girl" might mean a prostitute. Or, it could mean someone who misbehaves sexually in a way that the speaker disapproves of. Of course, slang being what it is, it could also be someone who misbehaves sexually in a way that the speaker _approves_ of. "Doing the nasty" is a slang expression that means "having sex."

The word itself isn't offensive, but that doesn't mean people won't get offended if you say they're nasty!