Alonzo Carter was sitting against a wall, covered in blood and smoking a cigarette, when police arrived at the Chester apartment occupied by Devanna Cornitcher and her two children on Friday night.
Three feet away from Carter lay the body of Cornitcher, 25, along with a bloody kitchen knife.
"I did it 'cause she tried to stab me," he told police, according to an affidavit.
On Saturday, police arrested Carter, 25, on charges of first- and third-degree murder, possession of an instrument of crime, and reckless endangerment of Cornitcher's two young children, who were at the scene.
Chester police responded to a call for a domestic disturbance, and were greeted by a neighbor, who had seen Cornitcher and Carter arguing in the stairwell. Cornitcher's 6-year-old daughter told them, "He killed my mommy." They proceeded upstairs and found a locked door; they could hear a child crying inside, according to the affidavit. The child was Cornitcher's 3-year-old son.