I think I explained the motivation just right. Wars are fought for national interests above all. Early involvement in Warsaw uprising was contrary to the Soviet interest. Was it moral or not is another question but they probably believed British-controlled Poland might be uncomfortable to have in the rear. I am saying it without taking sides right now.
That was the Nazi justification for invading the neutral Netherlands. "If we don't take it, the Brits will."
“I think I explained the motivation just right.”
The motivation for Katyn was to minimize any resistance to Soviet domination of Poland as a future client state.
The motivation for sitting on the east bank of the Vistula until the resistance in Warsaw was wiped out was to minimize any resistance to Soviet domination of Poland as a future client state.
Were there any other motivations in the latter case? Of course there were, as there have to be in anything complex and cutthroat as war, and one might even have a modicum of understanding of those even in the case of operatives as amoral as the Soviets - if they hadn’t encouraged the uprising in the first place.