In recent months Nikolai Glushkov had been in good spirits. True, he had to go into hospital in December from an operation on his foot but remained characteristically cheerful, returning to his home in Kingston, south-west London, and hobbling round on crutches.

Charming, with impeccable English, and debonair, Glushkov was well liked. He gave large parties attended by his friends and their children and chatted to his neighbours. His death last weekend was entirely unexpected.

But his daughter Natalaya, in London, and son Dmitry in Moscow had suspected foul play immediately. On Monday Glushkov didn’t show up to a court hearing at the commercial Rolls Building in London. He had spent months preparing for the case. He was due to defend himself against long-running claims of fraud from Russia’s state airline Aeroflot. When he didn’t appear Natalya went to investigate. She found her father at home, dead and with strangulation marks on his neck. The scene was disordered.

On Friday police confirmed he was murdered. Why? Glushkov was a close friend of the late oligarch Boris Berezovsky. They worked together in Russia in the 1980s and 1990s, with Glushkov becoming Aeroflot’s chief financial officer.

When Berezovsky fell out with Putin and escaped to London, Glushkov was arrested in Moscow. He finally got out in 2004 and joined his friend in exile.

The two were close. Berezovsky would telephone fellow oligarchs when Glushkov was behind bars, begging them to help. Glushkov believed he had been taken hostage so that Berezovsky would sell his ORT TV station to state interests.

When Berezovsky was found hanged after losing a court battle against Roman Abramovich, Glushkov was convinced this was murder. “The deaths of too many Russian exiles are happening,” he said.

Another victim was Alexander Litvinenko, whom Glushkov knew well. Glushkov investigated the case, talked to journalists and visited the Guardian office in London – a well-groomed man in late middle age, with silver hair, and carrying an unshakable grievance.

He believed the British authorities were failing to take these deaths seriously and to punish the perpetrator in the Kremlin. His murder seems to prove his point.

Timeline Poisoned umbrellas and polonium: Russian-linked UK deaths Show Georgi Markov In one of the most chilling episodes of the cold war, the Bulgarian dissident was poisoned with a specially adapted umbrella on Waterloo Bridge. As he waited for a bus, Markov felt a sharp prick in his leg. The opposition activist, who was an irritant to the communist government of Bulgaria, died three days later. A deadly pellet containing ricin was found in his skin. His unknown assassin is thought to have been from the secret services in Bulgaria. Alexander Litvinenko The fatal poisoning of the former FSB officer sparked an international incident. Litvinenko fell ill after drinking tea laced with radioactive polonium. He met his killers in a bar of the Millennium hotel in Mayfair. The pair were Andrei Lugovoi – a former KGB officer turned businessman, who is now a deputy in Russia’s state Duma – and Dmitry Kovtun, a childhood friend of Lugovoi’s from a Soviet military family. Putin denied all involvement and refused to extradite either of the killers. German Gorbuntsov The exiled Russian banker survived an attempt on his life as he got out of a cab in east London. He was shot four times with a silenced pistol. He had been involved in a bitter dispute with two former business partners. Alexander Perepilichnyy The businessman collapsed while running near his home in Surrey. Traces of a chemical that can be found in the poisonous plant gelsemium were later found in his stomach. Before his death, Perepilichnyy was helping a specialist investment firm uncover a $230m Russian money-laundering operation, a pre-inquest hearing was told. Hermitage Capital Management claimed that Perepilichnyy could have been deliberately killed for helping it uncover the scam involving Russian officials. He may have eaten a popular Russian dish containing the herb sorrel on the day of his death, which could have been poisoned. Boris Berezovsky The exiled billionaire was found hanged in an apparent suicide after he had spent more than decade waging a high-profile media battle against his one-time protege Putin. A coroner recorded an open verdict after hearing conflicting expert evidence about the way he died. A pathologist who conducted a postmortem examination on the businessman’s body said he could not rule out murder. Scot Young An associate of Berezovsky whom he helped to launder money, he was found impaled on railings after he fell from a fourth-floor flat in central London. A coroner ruled that there was insufficient evidence of suicide. But Young, who was sent to prison in January 2013 for repeatedly refusing to reveal his finances during a divorce row, told his partner he was going to jump out of the window moments before he was found.

Skripal poisoning Sergei Skripal and his daughter Yulia were were found unconscious on a bench in the Maltings shopping centre in Salisbury after 'suspected exposure to an unknown substance' which was later identified as chemical weapon novichok. In the aftermath Theresa May blamed Vladimir Putin and expelled 23 Russian diplomats who were suspected of spying. Two Russian men using the identities Ruslan Boshirov and Alexander Petrov were named as suspects. They appeared on Russian TV to protest their innocence.

The Skripals survived. However a local woman, Dawn Sturgess, died after spraying novichok on her wrists from a fake Nina Ricci perfume bottle converted into a dispenser, which had been recovered from a skip by her partner Charlie Rowley.

Newspaper photos of Glushkov with Berezovsky are poignant. The pair – together with Litvinenko and the Georgian tycoon Badri Patarkatsishvili – all sought refuge in London. All died in the UK under opaque circumstances.

Alex Goldfarb, who knew Glushkov well, described him as a talented manager, with a good sense of humour and “cynical irony”. Glushkov enjoyed wine, dressed dandyishly and was unfailingly generous, buying tickets for the opera at Covent Garden once a year for all the staff who worked in Berezovsky’s office.

He has a large ridgeback dog at home. “The dog was huge and fiercely loyal. I don’t know how his attacker got past the dog,” Goldfarb said. “They must have neutralised it”.

Glushkov wasn’t an oligarch or fabulously rich and never had a share in Berezovsky’s business. He “had a couple of million dollars”, Goldfarb said.

Glushkov was convinced from early on that Andrei Lugovoi had killed Litvinenko. “He was radical. I loved the man,” Goldfarb said of Glushkov. “Of the whole Berezovsky crowd he was the most pleasant to have around.”

Glushkov’s ex-wife is in Moscow. They remained on good terms. His family are devastated. “I’m practically the last one left,” Glushkov told the Guardian in 2013, in words that now seem grimly prophetic.