From their point of view, occupying the eastern part of Poland was logical. The move created a buffer Germans would have to attack first. Hard to argue with this.
If they wanted to have a buffer they would rather support Poland against the German invasion. I would argue that they wanted to remove the 'obstacle' for their future invasion on Europe.
I would agree with Victor Suvorov on this one:"Suvorov challenges the view that Adolf Hitler and the Nazi regime attacked an unsuspecting USSR on June 22, 1941 with a much superior and better prepared force. Instead, Suvorov argues that the Soviet Union was poised to invade Nazi-controlled territories in July 1941.
Suvorov claims that Stalin successfully manipulated Hitler into removing the "buffer zone" (Poland) between Europe and the USSR. Suvorov further argues that Stalin's goal was the export of communism to other countries"
The fact that many historians criticize his thesis but not the facts and numbers cited in the book adds to his credibility.
Just because Hitler was a bloodthirsty maniac doesn't make Stalin a peace loving animal. They both wanted to expand their rule over Europe. The difference is that the USSR has "won" and their narrative became the standard version of history tough in schools.