"My philosophy (even today) is that there exists an evil that cannot be explained – a virulent, terrifying evil – and humans are the only animals to possess it. An evil that is irrational and not bound by law. Cosmic. Causeless. Nothing frightens people more than incomprehensible, unexplainable evil."
That's Bergman on the film – suggesting that it has an almost Lovecraftian undertone. In fact a good way to read it is as a kind of existentialist horror film. We get some rather unsettling images of the carcasses of mutilated farmyard animals. The culprit is unknown, underlining Bergman's emphasis on the inexplicability of human evil. But the gruesome acts instill a sense of fear and suspicion in the community that spurs a local gang to find and punish a scapegoat. The account of the punishment is delivered in dialogue, mainly because it's too awful to depict visually. Anna's description of the death of her husband and son in a car accident is also grim going. Passion is not as hard to watch as Shame, which was made at the same time and shares many of the same preoccupations. But it's uncompromisingly unpleasant nonetheless.
My emphasis on it being an existentialist horror film is not flippant – in one of the key monologues towards the end, our protagonist Andreas describes the humiliation of failure and how the inability to assert himself against the world leads to a withdrawal from society. He shares his malaise with Ava, who is trapped in a marriage with a rich, aloof and sarcastic husband, and is also unable to pick and fulfill her own life project (she refuses to have children after a miscarriage). The husband is probably the most well-adjusted character, but he's also a bit of a creep, unconcerned by his wife's infidelity and subtly driving Andreas into debt and servitude. He's an amateur photographer who understands that photography cannot capture personality, just surfaces. That outlook is underlined (and perhaps undermined) by the insertion of four interviews with the four leading actors separately discussing their characters.
But clinging to illusions turns out to be even worse than disillusionment. Andreas's violence is the result of frustrated self-loathing – the realisation that he is a worthless human being. Anna cannot process her own failures, and instead fabricates a fairytale of her happy previous married life. Although she insists on the need for people to find a truth they can believe in and live up to, that imperative turns out to create hostages to fortune. If life doesn't comply with your truth, you change it, violently if necessary. Better to kill your family and remember them lovingly than go through the pain of seeing that family crack under the pressure of real life.
One of the ideas Bergman was playing with when making Passion and Shame was of Fårö (the island where he shot many of his films) as "the Kingdom of Hell". Although Bergman prefers to gloss the evil in Passion in a transcendental way, you can also read it as a malaise caused by the subjugation of the individual by society. Anna must cling on to her belief in her perfect marriage because of the ideals and expectations that surround her. Likewise, Ava is unable to escape her lofty and remote husband because she is forced to be a jewel in the crown of his many achievements. Andreas is a failure – he can't even fix his house properly (as the opening scenes make clear). He's not the embodiment of what we expect of the male hero.
The interviews with the actors is one way of getting to the notion that these characters are playing roles foisted onto them by the tyranny of society. The darkness of human hearts is not put there by an incomprehensible creator, but by a director making a film. And one way of surviving is to recognize that these are roles to be inhabited when needs must, and then cast off when you find something better. This is not something Bergman allows for here (interestingly, he now thinks inserting the interviews was a mistake). In Passion, nothing better is available, and the characters end up walled off against each other, wandering alone in a barren landscape. Human interaction is a recipe for hypocrisy, which leads to either delusion or nihilism, with violent consequences. Genuine communication is only possible once our propensity for roleplaying is recognised. In Passion, we only get to see it when the actors speak about their characters – when they step out of their role and have the freedom to reflect on it.