Gregorius, by Bengt Ohisson trs Silvester Mazzarella

"But why, of all people, must I keep running into Rev Gregorius? That there should be such people in the world! If, by pressing a button in the wall, I could kill that clergyman, I do believe I should do it." So thinks the physician-narrator of Hjalmar Söderberg's classic novel, Doctor Glas (1905), and kill Gregorius he does. He has listened in his professional capacity to that ageing pastor's beautiful young wife passionately inveighing against her husband's sexual demands, so particularly repugnant now she has a lover. This enables him to view murdering Gregorius as a positively humane act. Certainly Glas never feels remorse for it.

But, argues contemporary Swedish novelist Bengt Ohlsson (for whom Doctor Glas "is the novel I return to when I periodically lose faith in literature"), should we ever thus dismiss a fellow-human as expendable? Söderberg's novel takes the form of Glas's diary. Its very first entry establishes his hostility to Gregorius. Before we know where we are, we too are finding the pastor a disgusting, importunate creature of whom the world would be best rid. Ohlsson tells us in his fascinating afterword to his own book that the more inward he became with Doctor Glas, the more Gregorius asserted his individuality. Rereading his admission to the doctor that even, in late middle age, he wanted to be a father, Ohlsson knew he must write a novel from the clergyman's point of view.

Most readers, whether Swedish or British, will approach Gregorius out of admiration for Söderberg's disquieting masterpiece, taking it as its complement, in which the repulsive murderee addresses us – to reveal himself as different from how we have supposed. The ground between the two works should be cleared forthwith, to prevent any needless disappointment. Glas himself, so complex and mysterious in Söderberg, makes only scant appearances in Ohlsson. Indeed those ignorant of the original may not appreciate the terrible web he is weaving: providing Gregorius's wife with medical excuses for sexual abstinence, diagnosing non-existent illness in the pastor himself.

Then, in Ohlsson we see, though never meet, the wife's lover, but Gregorius finds out about him, as in the original he does not. Margaret Atwood's preface and Ohlsson's afterword are both illuminating, but I have to dissent from their view that Söderberg's Glas committed his crime essentially for love of the appealing Mrs Gregorius. Glas expressly compares himself to Dostoyevsky's Raskolnikov, and his deed – with which Ohlsson's novel ends – anticipates Existentialism's rejection of conventional morality. Söderberg gave us Glas to arouse our critical faculties rather than our sympathies; after all he himself was later a courageous opponent of Nazism with its denial of compassion.

Gregorius is anyway an autonomous work, in which the pastor appears a feeling, decent and wronged man. Faithful to the preoccupations of the time in which it is set – the warfare between religion and science, with both sides more emotionally engaged with the other than they cared to admit – it presents, in patient detail and with vivid tableaux of contrasting couples, a problem that then exercised many. If marriage is a sacrament ordained by the God Gregorius serves, why are the motives behind it so frequently mixed ones, and why does it ignore the fact that the very personalities of the partners can change? If sexual attraction is a simple biological fiat, how come other forces, societal, cultural, even psychic, can crush it so effectively?

Ohlsson movingly depicts the plight of a man aware he is physically abhorrent to the woman he loves but unable either to confront this or to deny himself the rights he presumes. But this is a merciful portrait of the pastor. At the spa, to which, as in the original, Glas advises Gregorius to go, he meets a married woman. Anna not merely admires him but falls in love with him, even though he is old, awkward and malodorous. And Gregorius loves Anna – but his response is damagingly undermined by the painful ordeal of his own marriage. "And one's life sits there, like a pair of shoes in the hall, never used," he thinks, and, in doing so, wins us over to him: a true victory for compassion as the indispensable virtue.

'Doctor Glas', trs Paul Britten Austin , is published by Harvill