In court on Friday the man responsible for killing the 51-year-old mother-of-four was told he must serve at least 16 years in jail for Marion's murder.
Fred McClenaghan, 49 and from Broad Street in Magherafelt, had a violent history - including convictions for assault and domestic abuse.
Three police officers have been disciplined for failing to act over a threat McClenaghan had a gun.
Speaking on UTV Live Tonight, Police Ombudsman investigator Jeff Smyth said it was time for the police to say sorry.
"I know that through our investigation that the officers that were interviewed expressed their regret in what has happened," said Mr Smyth.
I do think that on this occasion we would recommend to the Police Service that they consider issuing an apology for their failures and serious failures in this particular case.
Ombudsman investigator Jeff Smyth
McClenaghan walked into the laundrette armed with an antique shotgun in March 2011 and shot Marion Millican in the chest.
He had denied murdering his former partner after she ended their relationship.
McClenaghan was handed an automatic life sentence for murder and possession of a firearm with intent - and the 16 years is the minimum time he must spend behind bars before being considered for release.
But a daughter of the victim has hit out at the 16-year tariff, which she said was highly lenient.
Suzanne Davis said the sentence left the family feeling let down by the system.
She told UTV: "As a family we feel the sentence was very lenient, especially for such a crime, such a violent crime that has been committed. Sixteen years is definitely not enough.
"He is an evil, evil man, there are no words to describe the person that he is... a monster."
There were warning signs that McClenaghan wanted to kill his former lover.
He had tried to strangle her before and spoke of wanting to murder Marion - her sister even told police that he had a gun and was a danger.
Investigators were called in and found officers failed to act on that threat and others.
They didn't and weren't proactive around looking for any weaponry he may have possessed, his house wasn't searched and he wasn't spoken to by any police officer.
Ombudsman investigator Jeff Smyth
Mr Smyth continued: "Our investigations revealed that police had not called to speak to Mr McClenaghan, we don't believe it was risk assessed.
He said that while there were failings, the tragedy that unfolded could not have been foreseen.
"Undoubtedly these were serious failings," the Ombudsman officer said.
"There's no question about that. But I think we must also recognise that the police service themselves, in investigating the information concerned that was passed on to them could not have foreseen the tragic, tragic outcome for Marion Millican in this case."
Marion's family said their lives have been ripped apart my McClenaghan's actions, and believe a judgement call could have been the difference between life and death.
Suzanne said: "If you even call discipline still wearing their uniforms, still in employment by the PSNI ... that definitely is not discipline, definitely not. These officers should have lost their jobs.
"My mother lost her life and they still are employed by the PSNI."