Actress Dominique Dunne's Killer Boyfriend's Ex Details Domestic Violence Jurors Didn't Hear

Dominique Dunne was a rising star -- delivering the iconic line in "Poltergeist" -- before her life was tragically taken at the hands of her then-boyfriend ... and there's a woman who was not allowed to testify who could have changed the outcome of his trial.

The decades-old case against John Sweeney -- the man who strangled the young star back in 1982 -- is being reexamined anew in a documentary produced by TMZ Studios ... which puts a fresh light on the trial, the aftermath .... and presents shocking new evidence.

At the time, prosecutors charged Sweeney with first-degree murder case. He was convicted of manslaughter, and he was back on the streets after only 2 1/2 years behind bars.

The jurors you'll hear from in the documentary were stunned when we showed them our interview with Sweeney's prior girlfriend, Lillian Pierce, telling us had they heard from her they would have thrown the book at Sweeney.

Pierce was not allowed to testify during the trial ... the judge felt it would be unfair to Sweeney if she explained how he had repeatedly brutalized her. Pierce has never spoken publicly about the abuse she suffered at the hands of Sweeney, until now.

Check out the video ... what Pierce went through was horrific, and it mirrors some of what Dominique experienced.  She says Sweeney was wired to attack his girlfriend when he got mad.

What's more, we found yet another girlfriend whom Sweeney brutalized AFTER he was released from prison.