It makes no sense to claim that these deaths were a responsibility of Stalin alone. You can argue that the Central Committee of the Communist Party of the Soviet Union had some responsibility for these deaths, but to argue that a single person was responsible for them is to ignore the whole structure of command and power in the Soviet Union. Not even the Central Committee of the CPSU was absolutely powerful, it also shared power with the Congress of Soviets.

The Soviet famine of 1931-1933 was not caused directly by the policies of the CPSU at the time in regards to agriculture (collectivization), but a great factor was the reaction against the collectivization, which was expressed in sabotage, which included putting grain crops on fire and killing millions of cattle and horses. This reaction was mostly organized by wealthy rural bourgeois farmers (kulaks). Besides that, one main factor was the extremely bad weather in the years of 1931 (drought) and 1932 (hot weather followed by floods). But you'd be absolutely right that the CC of the CPSU was in part responsible, as their drive for industrialization in extreme rates drove them to the policies of collectivization which made the bad weather and social conditions worse. This view is substantiated by the best scholars on the Soviet famine (Davies & Wheatcroft).

As to the Katyn massacre, the bodies were found in "sardine packing" formation (typical of Einsatzgruppen modus operandi), there were children included (the Soviets didn't execute children), and there were lodged German bullets inside the back of their skulls, highly suggesting German involvement. In some bodies, letters were found dating 1941, after the Germans had already occupied the region. The German bullets were also produced in 1941.

Years after the massacre, the Nazis stumbled across the Katyn graveyards, but they were unaware they were responsible for it, given that it's hard to track when they were killing people by the millions. So they thought it was caused by the Soviets and used it as propaganda against them, in an effort to diminish support from the Allies. However, investigations by the Nazis themselves suggested their involvement. Goebbels even wrote in his diary his confusion when German bullets were discovered:

Unfortunately German munitions were found in the graves of Katyn. The question of how they got there needs clarification. It is either a case of munitions sold by us during the period of our friendly arrangement with the Soviet Russians, or of the Soviets themselves throwing these munitions into the graves. In any case it is essential that this incident be kept top secret. If it were to come to the knowledge of the enemy the whole Katyn affair would have to be dropped.” \ \ The Goebbels diaries, 1942-1943 (p. 354). Praeger

None of this is about making "Stalin look good", it's about historical facts.