They’re also rather sore that 18 years ago NATO fought to protect the Muslims who are now killing people all over Europe.
We were on the wrong side then and we still are.
It also humiliated what at the time was Yeltsin’s relatively pro-Western Russia by ignoring their historical ties to Serbia. That humiliation led to Putin, and the Russians deciding they needed to form a network of alliances to oppose American/NATO hegemony. Now we’re paying for it.