Oleg Sokolov, a Russian historian who made a career studying and impersonating Napoleon Bonaparte, liked to be called “Sire.” He also had a long history of seducing and being violent toward female students at a prestigious university.

“He thought he could do anything and looked down on the world around him as if he really were Napoleon,” said Lydia Nevzorova, the wife of a prominent Russian television personality who met Mr. Sokolov socially in St. Petersburg, Russia’s imperial-era capital.

On Monday, however, the once haughty Mr. Sokolov, 63, sobbed uncontrollably as he appeared in a St. Petersburg court to express “deep repentance” for killing and dismembering his 24-year-old student and lover, Anastasia Yeshchenko. “I am devastated,” he said.

On Saturday, Mr. Sokolov was fished out of the frigid Moika River in St. Petersburg — he had fallen in, drunk — along with a backpack containing the severed arms of Ms. Yeshchenko. A search of his apartment uncovered her decapitated corpse, and local news media said he had planned to dispose of his victim’s body parts in the river and then commit suicide, dressed as Napoleon, outside a St. Petersburg fortress.

The gruesome saga, while echoing the dark passions of Russia’s second city explored by the 19th-century novelist Fyodor Dostoevsky, has commanded huge attention in Russia as a very modern tale of crime and impunity.

As former fans, including members of a military history society sponsored by the Kremlin, scrambled to distance themselves from Mr. Sokolov, Russia grappled with a troubling question: How did a man dogged for years by detailed accusations of violent bullying manage to keep his job as an assistant professor at St. Petersburg University, President Vladimir V. Putin’s alma mater and one of Russia’s most prestigious academic institutions?