Athaliah was the only female monarch of the Kingdom of Judah, the only female monarch for all of Israel (see Kings of Israel and Judah, also the Fact Finder question below). Being female was the only thing that made her different; her cutthroat behavior, and her own demise, was the same as many of the kings of Israel and Judah who came before and after her. She assumed power through the murder of all but one of the royal princes, before she was herself assassinated on the order of the High Priest.
Athaliah was the daughter of King Ahab of Israel and his Baal-worshipping wife Jezebel, and the granddaughter of King Omri of Israel. She married King Jehoram of Judah. Upon the death of both her husband (who died from a God-inflicted disease because of his idolatry, i.e. 2 Chronicles 21:12-20) and her son Ahaziah who became king after Jehoram (The Lord also had Ahaziah killed because of his unfaithfullness, i.e. 2 Chronicles 22:7-9), she had almost all of her royal grandsons murdered, and assumed the throne of Judah herself. Only one of the princes survived the slaughter, the infant Joash, who was rescued and given refuge in the Temple by Jehosheba, his aunt, who was married to the high priest Jehoiada. In so doing, she preserved that Messianic line from King David to Jesus Christ in Judah.
Athaliah held the throne of Judah for six years until Jehoiada the priest declared it was time to restore the monarchy to the lawful heir (the purpose of the monarchy was more than just political, a reality made obvious by the fact that the new king would be only 6 years old); the army supported him, rather than the evil queen:
"Now when Athaliah the mother of Ahaziah saw that her son was dead, she arose and destroyed all the royal family. But Jehosheba, the daughter of King Joram, sister of Ahaziah, took Joash the son of Ahaziah, and stole him away from among the king's sons who were about to be slain, and she put him and his nurse in a bedchamber. Thus she hid him from Athaliah, so that he was not slain; and he remained with her six years, hid in the House of The Lord [see also "My Father's House"], while Athaliah reigned over the land."
"But in the seventh year Jehoiada sent and brought the captains of the Carites and of the guards, and had them come to him in the house of The Lord; and he made a covenant with them and put them under oath in the house of The Lord, and he showed them the king's son. And he commanded them, "This is the thing that you shall do: one third of you, those who come off duty on the Sabbath and guard the king's house, another third being at the gate Sur and a third at the gate behind the guards, shall guard the palace; and the two divisions of you, which come on duty in force on the Sabbath and guard the House of The Lord, shall surround the king, each with his weapons in his hand; and whoever approaches the ranks is to be slain. Be with the king when he goes out and when he comes in."
"The captains did according to all that Jehoiada the priest commanded, and each brought his men who were to go off duty on the Sabbath, with those who were to come on duty on the Sabbath, and came to Jehoiada the priest. And the priest delivered to the captains the spears and shields that had been King David's, which were in the house of The Lord; and the guards stood, every man with his weapons in his hand, from the south side of the house to the north side of the house, around the altar and the house."
"Then he brought out the king's son, and put the crown upon him, and gave him the testimony; and they proclaimed him king, and anointed him; and they clapped their hands, and said, "Long live the king!"
"When Athaliah heard the noise of the guard and of the people, she went into the house of The Lord to the people; and when she looked, there was the king standing by the pillar, according to the custom, and the captains and the trumpeters beside the king, and all the people of the land rejoicing and blowing trumpets. And Athaliah rent her clothes, and cried, "Treason! Treason!"
"Then Jehoiada the priest commanded the captains who were set over the army, "Bring her out between the ranks; and slay with the sword any one who follows her." For the priest said, "Let her not be slain in the house of The Lord." So they laid hands on her; and she went through the horses' entrance to the king's house, and there she was slain." (2 Kings 11:1-16 RSV)
Fact Finder: Was Jesus Christ descended through the line of the Kings of Israel, or the Kings of Judah?
See Israelite Dynasties