A Tunisian migrant arrested for the brutal double murder of a couple in their eighties was driven to kill by hatred for the populist Freedom Party of Austria (FPÖ), police have announced.

Delivering vegetables on Friday to the home of his victims, a couple whom the Tunisian suspected of being involved with the FPÖ, he first strangled the 85-year-old woman before using a knife and club to murder her 87-year-old husband.

The man then set fire to the kitchen of the family home in Linz — where emergency services discovered the bodies of the couple whilst fighting the blaze.

The 54-year-old, who has lived in Austria since 1989, had long felt that as a foreigner and a Muslim he was unwelcome in the European nation, Kronen Zeitung reported, the newspaper noting the man had developed a growing hatred for society.

Charged and convicted in 2011 of committing animal cruelty, the Tunisian mistakenly assumed the man who reported him was an FPÖ deputy. He then began to bear a grudge against the anti-mass migration party, according to police.

From then on, according to regional police director Andreas Pilsl, the suspect blamed the political party for every negative experience in his life; for example when he felt he was treated insufficiently well at the unemployment benefit office, or when his welfare payments were cut.

The Tunisian, who had regularly delivered organic groceries to his two victims, turned himself in to the authorities shortly after the double murder, disclosing he had first thought about drowning himself in the Danube before deciding instead to go to the police.

Confessing his crime, he told officers he thought his elderly victims had links to the FPÖ, a claim Pilsl denied. Local media reports the couple had even given their killer financial assistance because the organic grocery store owned by his Muslim convert wife was struggling.

Declaring himself “deeply bewildered” by the brutal murders, Austrian Chancellor Christian Kern said: “Crimes like these destroy people’s confidence in the cohesion of our society, and so must be condemned the hardest.

“If people, as apparently happened in Linz, are being murdered because of their connections — whether real or imagined — to a political party, then it is vital that we take a stand against this,” he added, speaking in a broadcast on Saturday.