Jennair Gerardot, 48, broke into the house of Meredith Chapman, 33, and ambushed her as she returned home on Monday evening.
Investigators say Mrs Gerardot may have worn a disguise. She sent her husband texts revealing her revenge plan.
Both women were dead when police arrived in response to 911 calls.
Detectives said the gunwoman's body was found by the kitchen, not far from Mrs Chapman's. Two bullets had been fired.
"There's blood everywhere," said a 911 dispatcher in a police scanner communication, according to the Delaware News Journal.