Misusing bad and badly is a common grammatical mistake. The word bad is an adjective and should be used to modify nouns and pronouns. Badly, like most words ending in -ly, is an adverb and is used to modify verbs. The thing that trips most people up is that linking verbs such a to be and to feel take adjectives rather than adverbs.
Why do people use bad and badly incorrectly so often in their writing?