Last Night's TV - Sri Lanka's Killing Fields, Channel 4; Luther, BBC1

This film contains very disturbing images," warned Jon Snow at the beginning of Sri Lanka's Killing Fields. It would, he continued, depict "death, injury, execution and evidence of sexual abuse and murder". He was right too, though when the final credits rolled you couldn't help but feel that the worst lay somewhere off screen, less in the atrocities shown than in the moral debasement that had led to them being filmed in the first place, and the terrible banality of the conversations that went on as they were filmed. Two utterly distinct kinds of footage had gone into the making of Channel 4's account of the closing weeks of the war against the Tamil Tigers. Firstly, there was video filmed by refugees trapped in the appalling "no fire zones" established by the Sri Lankan government, footage knowingly recorded to document a crime. And then there was video recorded by the criminals, as a souvenir of their own barbarity. And grim as the former was, it was the latter that truly shocked and that provided incontrovertible evidence that war crimes had taken place.

Channel 4's film addressed a crime of omission as well – the failure of the international community to effectively protest against the treatment of civilians in the closing stages of the civil war. It began with the withdrawal of the United Nations from Kilinochchi, the Tamil capital in the north, after the Sri Lankan government had announced that it could no longer guarantee the safety of the UN mission, a move interpreted here as a premeditated plan to remove inconvenient witnesses. What followed was a lethal kind of kettling, as Tamil civilians found themselves squeezed between the ruthlessness of their own soldiers (who weren't above using them as a human shield) and the aggression of the Sri Lankan army. The "no fire zones" turned out to be a bloody joke, being repeatedly shelled. And the Tamils' makeshift hospitals were hit so frequently that they eventually asked the Red Cross not to pass on their GPS co-ordinates to the other side, fearing that they were being used for targeting rather than avoidance.

Nothing you saw in the first half of the programme could conclusively prove that charge, or confirm the belief that the Sri Lankans would pause after one shell and then fire another to kill the rescuers, though it did corroborate eyewitness descriptions of appalling conditions on the shrinking strip of land occupied by the Tamils. Because it was filmed by the victims, it's all too easy for the Sri Lankan government to argue that it represents only the chaos of an ugly war, rather than hard evidence of a war crime. But their only workable strategy with the film that followed, though, was to dismiss it as a fake, since it incontrovertibly showed Sri Lankan soldiers executing prisoners in cold blood. It hadn't looked fake to Channel 4's technical analysts, and I don't think it would have looked fake to any viewer outside the Sri Lankan High Commission. "These are our state property. Let's shoot," said an off-camera voice, as bound prisoners were murdered. "Is there no one here with the balls to shoot a terrorist?" yelled another soldier, impatient with his colleagues' irresolution in front of three kneeling prisoners. Most horrible of all was the ogling trophy footage of dead women stripped naked: "I really want to cut her tits off," someone muttered, "if no one was around."

The Sri Lankan government's only response to these disgusting documents has been to question Channel 4's "standards and fairness", presumably confident that there's no great appetite in the international community to pursue the matter. The failure of the UN Security Council to insist on an independent investigation is "inexplicable and morally quite indefensible," said Steve Crawshaw of Amnesty International. But it's all too explicable I fear, even if the explanation involves a squalid combination of realpolitik and self-interest. "Will they be failed again?" asked Snow at the end, over footage of Tamil civilians pleading for help. The answer is probably yes, and the fact that this angry and powerful film wasn't felt important enough to bump The Fairy Jobmother from an earlier slot might be counted as one symptom of the failure. We care, just not quite enough.

Real atrocities get edged towards the graveyard slots, but fictional ones can still command primetime. Luther – the BBC's dysfunctional detective series (the adjective applies to both nouns) – is back, with Luther having been cleared of murdering his wife. I think he's still brooding though, because his breakfast appeared to consist of a cup of a coffee and a game of Russian roulette, something of a kill-or-cure antidote to early-morning sluggishness. Having survived, it was off to work at a new unit devoted to Serious and Serial Crime, where he finds himself on the trail of an exhibitionist serial killer whose ambition is to remind London's jaded citizens "what it's like to be really scared". After killing two women with a flensing knife, he arranges for his third murder to be streamed live by webcam to Luther's office so that he can talk the hapless detectives through it. "He's taunting us," said Luther, demonstrating his intuitive grasp of criminal psychology. Every now and then, Luther goes off to flirt with Alice Morgan, the psychopath who has a bit of a thing for him and expresses her insanity by talking very slowly through a suggestive half-smile. It's all very silly, but fairness compels me to add that they'd come up with a very effective last-minute shock. My pen hit the ceiling.