LONDON—Nikolai Glushkov, a Russian émigré, lived alone in a weather-beaten row house in South London with an aging dog and a cat named Braveheart. It was the waning days of March, and he was readying himself for something big.

The onetime finance director of Russia’s flagship airline, he was preparing for a trial in a London court. He told friends it would prove his innocence of longstanding financial charges by Russian authorities and expose Aeroflot Russian Airlines as a front for Russian security services. The case could also prove embarrassing for President Vladimir Putin, by illuminating a piece of post-Soviet history the Russian government has tried to erase.

After running out of funds to pay his own lawyers, Mr. Glushkov, 68 years old, planned to represent himself, and had amassed tomes on British law and forensic accounting. “This case was his purpose in life,” said Georgy Shuppe, a friend and former business partner. “He was not going to give up.”

On the eve of a preliminary court hearing, Mr. Glushkov stopped answering his phone. When his daughter drove to his house to investigate, she found him inside, strangled to death with a dog leash. Later that night, dozens of antiterror police cordoned off the house and began digging holes in his yard.

In Russia, his death was portrayed by state-controlled media as a homosexual tryst gone wrong. British police are treating it as a murder investigation. In August, they posted video footage of a black van seen near his home the night he was strangled, asking anyone with information to get in contact.