When Robert Trigg was given a life sentence for the killings of two women this week, it was much too late. Not just because it came six years after the death of Susan Nicholson – who Trigg murdered in 2011 – but because in 2011, he should already have been convicted of the manslaughter of Caroline Devlin in 2006.

The deaths of both women had been declared not suspicious by the police on first investigation, despite evidence of Trigg’s controlling behaviour and history of intimate partner violence. It’s only because Nicholson’s parents sought justice for their daughter at their own expense – hiring an independent barrister and pathologist to reexamine the original pathologist’s report – that Trigg isn’t still at large, terrorising another woman in her own home, perhaps killing again.

Sometimes, of course, criminals exert great deviousness and the police have to exert even greater doggedness and ingenuity to catch them. That, however, is not the case here.

Trigg killed both women in the town of Worthing. Their deaths were both attended by Sussex Police. In both cases, Trigg failed to call the emergency services himself – in the case of Devlin, callously sending her 14-year-old son to find her body. Both Trigg and his victims were intoxicated at the time of death. Both women appeared to have been asleep with Trigg when they died, and both were found in an unusual position.

And though women who suffer violence at the hands of their partner often cover for him out of shame and fear, Sussex Police had no excuse for not knowing what Trigg was. They had been been called to incidents involving Trigg and Nicholson at least six times before her death. In March 2011 – that’s a month before he killed her, so hardly in the lost and distant realms of history – Trigg received a caution for punching Nicholson in the face.

In fact, the explanation that Trigg offered, and the police accepted, for Nicholson’s death now sounds so pathetically unlikely, it’s hard not to become hysterical with outrage. He claimed that he rolled on to her in his sleep, inadvertently suffocating her.

As a CPS representative pointed out after Trigg’s conviction, with a dry understatement that says more than the most howling fury could: “It was extremely unlikely that two of Trigg's partners had died of natural causes while sharing a bed with him.” When the same terrible thing happens twice, we might reasonably start to suspect something more than bad luck.

There can be no justice for women if the police are failing to take the most blatant evidence of men’s violence as the starting point for an investigation. There can be no justice for women if their killers have to be pursued by private means: who will see your death answered for if the police call it an accident and your parents lack the means, ability or inclination to take on the case themselves? Justice that becomes a luxury good is no justice at all.

We tend to recognise now that women face a huge burden of disbelief and undermining when they try to call men to account. What we don’t perhaps talk about is the other side of the scale: the enormous positive discrimination they enjoy which means the same man in the same town can be found by same police force beside the body of a second woman, and say it wasn’t his fault, and have his lies accepted. It’s already too late for too many women.