It is doubtful that anyone who ruled the Soviet Union was a true communist. They were power hungry dictators. The same is true for those that served them. Just as most Republicans in the US are actually opposed to a "republican" form of government.
This would be especially true for the KGB and other types of secret police organizations. They were not attracted to the jobs and successful in them because of a devotion to a political philosophy. They were successful because the organizations capitalized on mean, evil and greedy personality traits.
As for why they want dominance over Eastern Europe, a barrier between them and Western Europe is part of it. Two invasions by Germany and one by France still haunt them. Based on American actions in the 50's and 60's, not a lot of trust of us either.
The earliest Russian Czars wanted more which is how Russia became as large as it did. The Soviets merely continued this philosophy. Think USA and Manifest Destiny and taking California and the West from Mexico. Running Spain out of Florida and then later Puerto Rico, etc. Pressuring England to sign over Northwest to US. Not to mention the Native Americans.
Putin wants to restore Russia to the glory it had when it was its largest. Remember, Russia today is much smaller than the Soviet Union for which Putin worked so diligently.
It is called Empire Building. For Putin, step one is domination and step two is reincorporation into the Motherland.
Poland was once a part of Russia, as were a lot of other now independent nations.
The Berlin Wall was erected to keep people in the Soviet Block from escaping to West Berlin and then to Western Europe. It was a wall in Berlin, but a fence or walls pretty along the entire border.
Don't ignore that "Nationalism" and threats from an enemy are two of the principles bad leaders use to distract the people from how bad things are in the country. Also permits them to do many illegal or borderline illegal things to accomplish their personal goals and the goals of those who support them.