This graph does not say that no-one is corrupt, correct. It does however show that the soviet system had much less inequality than what came before (under the Tsar) and after (capitalism). This is an improvement. This graph does not prove corruption either. Some having more than others is not corruption.

The soviets did not reach communism, they were building socialism.

Under capitalism, the vast majority of people must labour, by getting a job… if they can, to get money to have a house, food, medicine, etc. They take actions in line with how capitalism functions, to the extent they are doing so to survive, this is “human nature”, yes, but I don’t think this is the way that you are using those words. Under socialism, you are guaranteed a job, housing, food, there is free healthcare, etc. The actions the same person would take under socialism are different. So what you call “human nature”, but is just actions taken within context of capitalism, is not actually human nature.