I don't buy the blackmail idea.

Donald Trump is about power, about control. Trump admires the idea that if Putin doesn't like someone, that person has a way of leaving Russia, disappearing from public view, being in jail, or being dead. Trump likes the idea that if Putin wants something done, it's done. And the Duma makes it legal later.
Trump doesn't like the messiness of democracy. Trump wants to make offers people -- Democrats, congresspeople, judges, attorneys general, special prosecutors -- can't refuse.