The Florida woman had a row with her boyfriend.
Enraged, Adele Paula Bing, 52, pistol-whipped him in the head.
Before Mr James Lane, 39, left for the hospital on Monday afternoon, he warned that he would come back and kill her, Florida-based television channel WTSP reported.
So when Bing heard a knock at the door that night, she feared it was Mr Lane. She armed herself with a baseball bat and a gun, Mail Online reported.
But it wasn't him. It was her daughter, Ms Ruby Bing.
According to a police affidavit, as Bing was opening the door, she accidentally discharged her .22 calibre pistol, only to discover that it was her 25-year-old daughter knocking. Ms Bing was struck in the chest and collapsed while holding her four-month-old daughter in her arms. The child was not hurt.
A witness described to The Ledger seeing the young woman bleeding from the nose and mouth as she lay dying on the front porch.
Bing rushed to her side, cradling her daughter's lifeless body and begging her to "wake up".
Ms Sally Newcomb, 67, who heard the gunshot said she had a hard time believing a shooting had occurred the night before. Other neighbours agreed.
"It is so quiet around here," said Ms Tiffany Olaitan, 42. "I would have never though (this would happen). That's why it's got to be an accident.
"The worst that can happen out here is your neighbour has a good day and blasts his music."
CHARGED WITH MURDER
Bing has been charged with second-degree murder and child abuse without great bodily harm in the slaying of her daughter.
Ms Bing is survived by her newborn daughter and two sons. The younger boy was just days away from his first birthday.
"How can I look my grandkids in the face and say I killed their mother?" police quoted Bing as telling them. "You can lock me away for good."
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