Jael was called the “most blessed among women”. She was blessed among women in tents. Sisera asked for water and she gave him milk. She brought out cream in a lordly bowl. Sisera must have thought that he was safe because Jael seemed so accomodating.
It also says in Judges 4:17: However, Sisera had fled away on foot to the tent of Jael, the wife of Heber the Kenite; for ther ewas peace between Jabin king of Hazor and the house of Heber the Kenite. Sisera felt he could trust Heber’s wife because he thought she was on his side like her husband but he was wrong. She killed him and when Barak showed up she went out to meet him and said, “Come, I will show you the man whom you seek.”
Milk makes a person sleepy which was probably why Jael gave some to Sisera instead of the water he asked for. Plus, he was tired. Instead of dying in battle, Sisera died at the hands of a brave and resourceful woman.