Hamlet treats her cruelly because Ophelia gives him back his love letters, however he restrains until she does not tell the truth about her father’s whereabouts. He gets furious because she tells him lies on his face and loses his trust over her. All these tragic instances including his father’s death, his mother’s marriage to his uncle, and now his beloved lies to him, therefore, he feels lonely and abandoned by all people around him. In fact, his loneliness becomes the reason of breakup with Ophelia.
In Act III, Why Hamlet treats cruelly to Ophelia?