The thing with whataboutism is that (besides obviously just a trick to swamp discussion about any topic) is that even if you take it as face value it's often dishonestly out of proportion. Sure NATO did bomb Serbia and a lot of civilians were killed but how does that justify killing an order of magnitude more civilians in Ukraine (and counting)?

And if you want go down the whataboutistic rabbit hole, someone will bring up that Serbia played a major role in other conflicts of the yugoslav wars and that NATO was just reacting to a decade of calls for "why isn't the west doing anything to stop this?"

The thing that enables this sort of atrocities is not considering your target as fully human. American's way of coping with that is that they think most of the world is just a primitive shithole. Russian's way of dealing with Ukraine is that of saying that Ukraine just doesn't exist, that it's just a fake simulacrum of mother Russia, that its people are just jokes.

This setup helps fueling the asymmetric situation where grievances on your side are amplified and those in the other side trivialized.