SORRY! I mean use "badly" by strict grammar.

Thing is, "bad" <em>can</em> be an adverb, as a matter of fact, which means the same as "badly", like the "quick", "quickly" stuff. But use "bad" when you want to make your writing sound serious (I mean scholaristically), in an exam or something.

Badly is correct. As you are describing a verb (want), you should use the appropriate adverb (badly). 'Bad' is an adjective, and can only describe a noun. 

Best wishes

Besty