"That woman is bad."
Translation:Tamta kobieta jest zła.
Well, you just use different forms. "zły" is the masculine adjective, right? And then there are different forms: "zła" (feminine), "złe" (neuter singular and 'not masculine-personal plural)' and "źli" (masculine personal plural). The masculine personal form is usually quite different from the other ones, it's kinda... softened. "źle" is the adverb and clearly it's more similar to the masculine personal form here.