“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.
You might very well encourage uprisings without putting boots on the ground. US government does it quite often.
Venezuela is a recent example. US called for the uprising and supported it and some probably expected US troops to back the protests.
Is it a warcrime that the troops didn’t follow?