Earlier quoted context omitted.
But how do you know so clearly that the one thing is a lie and the other is the truth? Even if you reject the "encroaching NATO" narrative, what makes "Putin just woke up one day and decided that remaking the Soviet Union and/or the zarist Russian Empire would be a great thing to do in the 21th century" the more plausible hypothesis? What information do you have that Mearsheimer doesn't?
There are plenty of articles by respected international relations and Russia experts you can find that explains it quite clearly. The IR community does not share Mearsheimer's take. He is very much known to be the exception. Which is why he's the only one we're referring to by name here, because his analysis is so contrary to the overwhelming consensus of experts.
(Having both "pro-russian" and "pro-western" family members, so I'm engaged in lots of discussions currently and would be glad for new information, no matter which side)
I think it's important which countries the experts are coming from. That our own IR/Russia experts are sharing this view doesn't seem very surprising to me - it's a war situation after all. I just notice that a lot of non-Western countries seem to be at least undecided which narrative to follow, e.g. Brazil, India, Turkey, many African countries. (Ignoring China which is obviously allied with Russia and therefore also has a clear bias).
Also, BRICS membership seems to be in demand, what I wouldn't expect if it was generally believed to be dominated by an insane, warmongering megalomaniac.
Mearsheimer is not alone though (even though it's definitely a minority position here). Jeffrey Sax, Ulrike Guerot and, well, Noam Chomsky come to mind, or also organisations such as fair.org with well-documented sources.