Sadistically is the adverb. It tells you how someone does a thing: in a cruel way, and with enjoyment of the other person’s suffering. If someone smiles sadistically, they are not only being unkind; they like that the other person is hurting. It is a long, rather heavy word. In daily talk people more often say “in a cruel way,” “to be mean,” or they just use the adjective sadistic. You will meet the adverb in books, news, and careful writing more than in easy chat. Though actually, like sadistic, people sometimes stretch it for anything that feels too hard on purpose (“the questions were sadistically difficult”). That is looser than the darkest sense.

以虐待的方式；帶著施虐快感地