The 33-year-old made his pointed statement to journalists awaiting him at the airport as he returned from representing Argentina during the international break.
Messi's comments are most likely a response to Eric Olhats, the former agent of his Barca teammate Antoine Griezmann, following a scathing personal attack in the press.
Olhats told France Football: "Antoine arrived at a club with serious problems where Messi controls everything.
"He is both emperor and monarch and did not see Antoine’s arrival with good eyes.
"Messi’s attitude has been deplorable, he has made him feel it. I’ve always heard Antoine say that he had no problems with Messi, but never the other way around.
"It is the regime of terror. Either you are with him, or you are against him."
Messi has already been embroiled in a public civil war at Barcelona this season.
The Argentine forced the resignation of former president Josep Maria Bartomeu after repeatedly criticising the club's direction under his stewardship.
A new president is now set to be elected in January, but it remains to be seen whether Messi chooses to pen an extension to a contract that currently expires at the end of the season.