You're missing a small thing - NATO had had various plans for antimissile defences in countries in Central and Eastern Europe ( Poland, Czechia, Romania if memory serves me right). Those defences would potentially invalidate Russia nukes, and that's scary. ( Which is why there was a treaty limiting antimissile defences during the Cold War, to avoid one side thinking they can win a nuclear exchange). So Russia has plenty to be scared of, and then there's the imperialism, exporting problems, having a cause for the people to rally around, etc.

War crimes and abhorrent nonetheless. I hope at least this time the responsible end up in The Hague.