Jezebel was a fanatical worshiper of the heathen idol Baal. As queen of Israel, she attempted to force it upon the Israelites in the most ruthless and deadly manner. King Ahab made very little effort to stop her - quite the contrary, he actually built a pagan temple for her in the capital city of Samaria and allowed 450 prophets of Baal and 400 prophets of Asherah to eat at the royal table (1 Kings 16:32, 18:19).
Ahab and Jezebel were a clear example of why God prohibited the intermarriage, and in this case a politically-inspired marriage, between the Israelites and the "people of the land" at that time. It almost always resulted in the corruption of the Israelite, rather than the conversion of the idolater.
The idolatry was of course a direct violation of the First and Second of The Ten Commandments. In Jezebel's case, it involved some of the most vile and perverted behavior imaginable. Even little babies were sacrificed in fire as burnt offerings to her god of stone. It was the height of wickedness.
Jezebel directly attempted to overthrow the worship of God - she commanded that the people forsake The Lord, and murdered as many of His prophets that she could find (1 Kings 18:4). The remainder were forced to hide in caves in the wilderness (1 Kings 18:13). Whether through arrogance or ignorance, challenging The Lord was of course her and Ahab's most foolish mistake. It spelled the end for them.
First, all of Jezebel's prophets of Baal were killed after a confrontation with the prophet Elijah on Mount Carmel (1 Kings 18:16-46).
Then, Ahab's turn. During a battle, an enemy archer fired a very "lucky" shot from a distance, aiming only at the body of troops. What did he hit? Ahab - the random shot not only hit the king himself, but also just "happened" to hit him between the sections of his armor where there was no protection. He bled through the day before dying at sunset (1 Kings 22:29-38). In a very ghastly spectacle, dogs licked up his blood from his chariot - just as The Lord had declared for his wickedness (1 Kings 22:38).
Jezebel's own extremely horrendous fate then came. Jehu had three eunuchs throw her from an upper story window, where she was then trampled to bits by horses in the street. Dogs then came and ate her body, leaving only the skull, feet and hands (2 Kings 9:30-37), according to the word of Elijah (1 Kings 21:19). It was a horrible end for a wicked human being.
Fact Finder: What was the name of the man that Jezebel had stoned to death because she wanted his vineyard for a vegetable garden for her husband Ahab?
1 Kings 21:1-14