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.
The USSR wanted to decapitate the Polish leadership. If the USSR survived the war, its agents would be the only leadership left. If it didn't survive the war, it wouldn't matter.
The record of the wartime summits -- Tehran, Yalta, Potsdam -- supports the notion that Roosevelt was looking forward to the end of the war, Stalin looking past it, and as usual Churchill was the only person in the room who got the whole picture.
The USSR had a long history and a consistent pattern of preparing leadership cadres for other countries. They eventually had a formal institution -- Patrice Lumumba University -- for that express purpose. When the Japanese were driven from Korea, Kim Jung-Il returned from Moscow to his homeland. Ho Chih Minh spent most of the '30s and '40s in Moscow before returning to Indochina. Mao Zedong never spent time in Russia, but was inspired by its example and sought its advice.
In the chaos of post WWII liberation and decolonization, the communists were the best-organized and best-funded group on the ground in a number of countries, because they'd planned for it. it is not a stretch to believe that they were applying that strategy in Katyn in 1940. But for the Truman Doctrine and US efforts, they might have gotten Italy and Greece, too.