One Hundred Days, By Lukas Barfuss. Granta, £14.99

Genocide is sombre subject matter. In Lukas Barfuss's novel – its title refers to the approximate duration of the Rwandan massacres in 1994, which claimed more than 500,000 lives – in spare, devastating prose, the Swiss author captures the tragedy's unstoppable momentum.

His protagonist, David Hohl (the surname means "hollow"), recounts his experience to an old schoolfriend when he returns to his Swiss homeland, broken and disillusioned. Hohl was guided towards development work by a strong sense of justice. Initially Rwanda appears a benign, sleepy country – "the land of eternal springtime" – but he discovers it is also a corrupt dictatorship supported by foreign aid. Hohl is befriended by a dissolute colleague, Missland, who claims that the Rwandans have a hidden side, "an ugly, violent one".

As the massacres start, Hohl makes the quixotic decision to stay in Kigali. Soon he is stranded in his official residence as the smell of corpses begins to rise over the city.

In the aftermath, Hohl agonises over international responsibilities for the disaster. "We were blinded by our sense of virtue," he says. The question of whether misuse of aid for malign ends can ever be stopped is profoundly uncomfortable. Hohl reflects that Hutu extremists exploited the infrastructure gifted by their overseas benefactors to expedite killings: "We installed telephones for them, telephones they used to order murders."

The novel (translated by Tess Lewis) delves into the disturbing complexities of cruelty. Hohl is appalled to find himself sexually aroused by his Hutu girlfriend Agathe's violent racism towards his Tutsi housekeeper. Can proximity to abusive behaviour degrade us and, possibly, make us complicit? The issue can be extended to the media coverage of genocide – as we consume it, are we motivated by morbid fascination, as well as moral outrage?

Barfuss's debut novel has explicit parallels with Joseph Conrad's Heart of Darkness. If he cannot rise to Conrad's genius, his writing is seriously good, dramatising horrific events in illuminating ways.