Why is criticism, wrong or right, sinas chinam?
In addition to @Dan's answer, there's a huge difference between respectfully questioning a practice and branding said practice - done daily by thousands of Yidden - as being against halacha.
Now, add to that that the practice was instituted by a tremendous tzadik about whom there is no time or space right here to detail his literal mesirus nefesh for teaching Torah. This tzadik was then accused of breaking halacha "on a whim."
The amount of restraint on the part of forum members in response to that is admirable.
But on top of that, even had it been respectful questioning, not abhorrent hate mongering, the question was answered numerous times. The Tehillim is said after davenning, and the Frierdiker Rebbe even made sure to insist that a kaddish be said after davenning and before Tehillim to remove any illusion of it being a part of davenning. The name of the practice, as referred to by Chassidim and by the Frierdiker Rebbe in describing the practice is "Tehillim AFTER davenning."
The fact that he continues to double down on his absurd "changing nusach hatefillah" claims, even in the face of hearing Tehillim is recited after davenning in BMG, is more than telling.
There are also many other non Chabad shuls who add Tehillim for the land and/or residents of Israel, the troops, sick people and any number of causes and reasons. Yet they didn't "change nusach hatefillah on a whim".
Upthread I satirically said how Chabad replaced shema and shemona esrei with Tehillim. Patently absurd, right? Well, wouldn't you know, a few posts later that idea was posted in all seriousness as a criticism.