I think the basis of what Mearsheimer is saying - and why he is right to a degree - is that the West views its world order as the legitimate standard for governance and statecraft. Because of that, their idealism and ideology has become a series of speed bumps when, according to Mearsheimer's offensive realism, their adversary simply chooses to not play by their rules.

That is why the West is principally responsible, because they willingly ignored the signs and symptoms, and responded with no proactive policy. Ukraine (I might add Belarus as well) as a case study of Western policy has been a prime example of reactionary measures.

If you disagree, look at the original Ukraine Crisis and the seizure of Crimea. I think Mearsheimer would suggest that as well.