Im ok with trusting Medvedev on this point. As he should be an expert. He's said corruption is involved in 30% of the Russian economy. That i suppose are the Billions you were wondering about.What billions, specifically, has Putin stolen?
As for killing political enemies and stealing billions from the people, you've pretty much described the US political engine perfectly.
Russians are corrupt, no doubt about it. But it's out in the open and all Russians know it. Only Americans (like yourself) are stupid enough to think that it doesn't happen here.
I'm one of these strange people that thinks when an outspoken critic of Putin dies of Polonium poisoning or suddenly finds themselves imprisoned, that Putin may have had something to do with it. But that's just me. Smart people would know Putin had nothing to do with these little difficulties his opponents seem to get themselves entangled in. I imagine smart people such as yourself know that Putin is working day and night to find and arrest these assassins and go after those who have given false testimony.