Despite the biblical prohibition on sexual relations between half-brothers and sisters, (Leviticus 18:11) Amnon had an overwhelming desire for her. He acted on advice from his cousin, Jonadab the son of Shimeah, David's brother, to lure Tamar into his quarters by pretending to be sick and desiring her to cook a special meal for him. While in his quarters, and ignoring her protests, he raped her, then had her expelled from his house. King David was angry about the incident, but could not bring himself to punish his eldest son, while Absalom, Amnon's half-brother and Tamar's full brother, nursed a bitter grudge against Amnon for the rape of his sister.
Two years later, to avenge Tamar, Absalom invited all of David's sons to a feast, then had his servants kill Amnon after he had become drunk with wine. (2 Samuel 13)