I'd say a weakened or non-existent NATO makes the possibility of everything going boom as great as a direct conflict with Russia right now, it would just take a little bit longer as Russia would have much more incentive to keep going and less deterrent to him escalating to a nuclear conflict.
There are certainly incidents of Americans committing atrocities in the past, to include several in the last 20 years, but it has been over 50 years since the last unit-wide atrocity that approaches what has been discovered so far in Ukraine, and even in that case it was much more isolated than what has happened in Ukraine.
Again, the scale and scope of what has happened in Ukraine hasn't been seen in Europe for at least 30 years, if not 80. That it was so widespread, so casual and indifferent to the possible consequences it indicates much deeper and systemic issues in Russia than a few individuals who commit war crimes like we had in Afghanistan and Iraq. Coupled with the Russian indifference to targeting and the invasion itself I think the Russian leadership is much more reflective of the Russian people than you seem to think. Propaganda is a helluva drug.