Stalin spent a lot more time during the war thinking about after the war than the other Allies.
At Katyn, the Soviets killed the cream of the crop — the military officers who would be candidates for future leaders. And then, in the final push to Warsaw, the Red Army waited on the opposite side of the Vistula while the Polish resistance rose up against the Nazis; they decided to let their enemies kill each other off.
That way, the only Poles who were left with the skills and ambition to lead post-war Poland were the communist cadres Moscow had in its pocket.
And then, in the final push to Warsaw the Red Army waited on the opposite side of the Vistula while the Polish resistance rose up against the Nazis; they decided to let their enemies kill each other off.
1. I doubt that Stalin could have foretold the actual war events at Katyn's time. Probably he wanted to conquer Poland but hardly in the way it happened.
2. Soviet Union had a territory dispute over Western Belarus and Ukraine. At first Poland won a war and kept the territory for 20+ years. But the USSR got it later after Poland had lost WWII. The Polish POW who were probably killed by NKVD consisted of military, civil officers and colonisers. Those people were likely to rebel, so getting rid of them had political sense.