One of my favourite Gary Larson cartoons shows an audience of ketchup bottles sitting in a cinema watching a film that, naturally, stars bottles of ketchup. It seems to be a violent kind of film. On screen, one bottle lies smashed and bleeding red pulp all over the street, while another looks on, numb with horror. "Don't worry, Jimmy," a big ketchup bottle in the audience is telling a smaller one, possibly his son, "they're just actors – and that's not real ketchup."

The original version of this advice is worth heeding if, as a human being, you are watching other human beings in a film directed by Quentin Tarantino. Bullets rip into flesh, which flies out in messy chunks as blood spatters walls and falls like crimson rain on nicely arranged white flowers – on and on it goes, bang-bang-bang, spatter-spatter-spatter, mercilessly, while we, a grey-haired audience on a Wednesday afternoon, break open our foil-wrapped carrot cake and drain the dregs of our cappuccinos. From the moment we sat down, we've been softened up for these scenes. First came the Pearl and Dean adverts, and then the trailers, all of them an assault on the senses full of dizzying cuts and deafening noises, as if their makers wanted to pull you from your seat, slap you around the head and make you surrender to their juvenile interpretation of the world. (Which among us has ever seen a film because of its trailer? And how many films, probably some quite decent ones, have trailers put us off?)

And now comes Django Unchained, with its certificated warning to the unaware, or perhaps welcoming words to the aware, that it contains scenes of "strong bloody violence". It begins intriguingly (spoilers follow). In the antebellum American South, an itinerant German dentist (Christoph Waltz) frees the slave Django (Jamie Foxx) and together they form a bounty-hunting team that brings in wanted men dead or alive, which as Waltz explains is a "flesh for cash business", just like slavery. Naturally, there is a good deal of murder and unfeasibly accurate shooting, but it soon becomes clear that Tarantino's interest in historical verisimilitude doesn't stretch much beyond the word "nigger". "I kill white folks and get paid for it," says Django at one point. "What's not to like?" The Ku Klux Klan have terrible trouble fixing their hoods, which is a very funny scene, but also, in 1858, ahead of its time. As for the "Mandingo wrestling", one of the setpieces, in which two slaves fight to the death for their owners' amusement, Tarantino seems to have invented the ritual and borrowed its title from a 1970s novel and movie.

To chastise the director for fictions and anachronisms may be to miss the point. We know Tarantino's interest doesn't so much lie in history as in the history of films; not so much in the time depicted as in previous depictions of the time. Spaghetti westerns, blaxploitation movies, John Wayne: all get their salute as the two men change the purpose of their travels from the hunt for profitable criminals to the finding and freeing of Django's wife, who is still enslaved on a Mississippi plantation owned by the capricious Mr Candie (Leonardo DiCaprio). And yet Tarantino, to judge by his promotional interviews, wants it both ways: to create a preposterous entertainment and at the same time to have it taken seriously as a rewarding study of American slavery.

The same charge applies to his violence, of which, according to his now celebrated interview with Channel 4's Krishnan Guru-Murthy, there are two types. The first is what the owners and their assistants do to their slaves by way of whipping, manacling and having dogs tear them to pieces. The second is the catharsis unleashed in a series of bloody events, culminating in Django's massacre of the owner's entire household and the kneecapping of a loyal black servant – an "Uncle Tom" – whom we have been invited especially to dislike. That one avenger should kill so many people, themselves armed, in such a short space of time would, of course, have been impossible in the days of six-shooter pistols and awkwardly loaded rifles, but set a boy down in a school with a modern assault gun or two and it becomes much more possible, if a boy's thoughts of catharsis should run in that direction.

The school shootings at Newton, Connecticut, had happened only three weeks before Guru-Murthy began to ask Tarantino about the possible links between cinematic and real violence, which perhaps explained the director's thrilling loss of control: "I refuse your question … I'm here to sell my movie … I'm shutting your butt down." His view is that no link exists, that movie violence is a fantasy lacking any effect on real-world behaviour – unlike, say, the banned TV adverts for cigarettes that were also a kind of fantasy on their maker's part, though one the consumer could easily satisfy at any tobacconist's. The question may be complicated, but to deny any link absolutely is surely to protest too much, unless your reputation and the multi-billion-dollar industry you're a part of depend on it.

I suppose I don't "get" Tarantino. I'm not sure I completely fall for Martin Scorsese either, ever since I saw his original storyboard for Taxi Driver and saw that in his sketches he had coloured the blood red when everything else – faces, cars, suits – was grey, as though he had bitten his lip like a concentrating child and borne down heavily with a different choice of pencil; "more blood" read an instruction under one of the frames. Certain film critics also leave me at a loss. Insightful, sympathetic writers, such as this paper's Peter Bradshaw or the Observer's Philip French, accord Tarantino what, to me, is a mysterious degree of respect. "A powerful film, its dramatic brush strokes broad and colourful, its psychological points made with considerable subtlety and wit," French wrote of Django Unchained, " … it places Tarantino among the most impressive film-makers at work today."

How can that be? In its first two hours there are some good things, not least the script, and Waltz is thoroughly engaging as the film's wittiest and most humane character. But the subjects Tarantino finds consistently exciting are people being murdered, people screaming in pain, people begging for mercy. Boring might be the wrong word for these events, but their profusion takes away meaning. So much cruelty, so much noise, rehearsed and repeated until it satisfies the man-child in the director's chair. The credits carry a statement promising that no horses have been hurt in the film's making, and it comes as a slight surprise that they have been the only living things in real danger, other than our sensibilities and imaginations.

The next day I saw a documentary about the photographer Don McCullin, whose pictures of wars and disasters have informed so much of our understanding of the world over the past half-century. In the film, his pictures from Biafra, Vietnam, the Congo and Beirut are supplemented by some disturbing and rarely seen footage from the same conflicts. Heart-breaking cruelty, no slow-motion blood: the sight of real violence tends not to inspire or seduce.

• This article was amended on 29 January 2013. It originally misspelled Christoph Waltz's name as Christopher Waltz. This has now been corrected.