The US is not a democracy, and never has been. The political system has always been corrupt and for sale to the richest individuals. The difference Citizens United made was to introduce rich corporations into the mix, effectively kicking out all but the very richest individuals, but also broadening the impact of the corruption by effectively broadening the pool of "players" (because there are even more incredibly rich corporations than there are incredibly rich human individuals).
I don't think the Russian interference is a result of CU. It's just the culmination of the degeneration of the system. The pernicious Republican cheating in elections, which has been obvious for many years now, has demonstrated that the system, such as it was, couldn't protect itself. If the Russians could find an idiot that they could fund - and thus control - to run for president, they would own the American government. And the Republicans helped them do it.