Denis Nikitin arrived for an interview at a Kyiv restaurant with a pistol strapped to his side and flanked by two armed bodyguards. It would not have been a surprising sight in wartime Ukraine but for one detail: Nikitin is Russian. Before Vladimir Putin’s invasion of Ukraine last year, Nikitin was known as a notorious Russian nationalist, who built links between far-right groups across Europe and was once a major figure on Russia’s football hooliganism scene. “If we had met 10 years ago on the outskirts of Luton we might have had a fight. But now I’m a grownup,” he...