According to the criminal affidavit ... Mollie Fitzgerald repeatedly told cops in Johnson County, Kansas she had to kill her mom in self-defense last month because her mother, Patricia, came at her with a knife.

In docs, cops say Mollie -- who had a small role in "Captain America: The First Avenger" -- told them she had to disarm 68-year-old Patricia, and during the struggle ... stabbed her 4 times in the back. When cops arrived, they found Patricia in a pool of blood with a knife lodged in her back.

Meanwhile, medics said Mollie had a series of small cuts on the palms of her hands and a bite mark on the inside of her left bicep. She was treated on scene with a tetanus shot and bandages.

But, according to the affidavit, an autopsy revealed it was Mollie's mother who was defending herself from a knife attack -- she had wounds on both of her hands described as "defensive injuries." One of the wounds nearly severed her thumb from her hand. She also had bruising on her face, lip and neck.

The Medical Examiner "did not find any injuries consistent with Patricia having delivered any strikes or having been in control of the involved knife." Based on that info, cops determined Mollie was the aggressor and booked her for murder.