Mearsheimer completely ignores the agency of small European countries and makes the classic IR mistake of assuming that what might be 'rational' behaviour for a state in a Realist sense is also correct behaviour.

You can use Realism to predict that Russia would eventually try to regain the power it lost over Eastern Europe and the Baltics with the dissolution of the USSR. In fact that very understanding and fear is precisely why countries in Eastern Europe and the Baltics pushed so hard to join NATO and the EU, because they predicted that would happen.

But that doesn't make Russia's actions the right move, either legally or morally. They have agency too and could always have made the choice to not to try to act like an empire with violent hegemony over their neighbours.

1. I did not hear him equate realist with moral in that talk. Most people would consider the realist school to be amoral. (I advocate neither)

2. "NATO pushed..." does not imply Russia is right. Russia is wrong, legally and morally, but it can be simultaneously true that NATO pushed Russia.

1. He has in other talks, and he has used it as a way to explain away Russia's actions. As though they're an actor with no agency that can only react.

2. "NATO pushed" is deceptive framing. The countries that joined NATO after 1991 had to lobby hard over many years to get NATO members to agree to expand. Even then the process took forever with MAPs and other interim steps each designed to slow the process down.

These people believe that only the USA makes real decisions out of free will and everything else is only a consequence of those decisions or others uncontrollable reactions to those decisions. Apparently being an expert in geopolitics is mainly just about having a story and sticking to it through thick and thin.