That being said, I think on the topic of Russia and Ukraine, Mearsheimer is not consistent with his own theory (defensive realism). I completely understand why Putin is doing what he's doing, his actions are very well explained by Mearsheimer's framework; he's not a madman (necessarily). But to say that what Russia is doing is somehow US's fault, that really does not make any sense.
US and NATO do whatever is in their best interest, and that implicitly means they will try to reduce Russia's power. Russia knows that. In turn, Russia will try to reduce NATO's and US's power (and increase their own). That's all rational. Now, to say that the US and NATO have to have some form of guilt, and they should change their actions, that's crazy, it does not make any sense.
To say that the US, or NATO, or Russia, or anyone else is hypocritical, or they say one thing and do another, well, welcome to politics. "Not one inch", said NATO some decades ago. Russia calls foul play. Well, what did they think? People say one thing and do another thing all the time, that's the way you play the game.
But, one thing is to not keep your word about this or that, another thing is to start killing people. By the thousands. And destroy their homes. And make them leave their countries. Is really Mearsheimer incapable of understanding this difference?