Ligne Chen isn't the type of antagonist who has a giant freeze ray he'll turn on the world unless he's paid one billion dollars. (Unfortunately. We love song and dance.) Nope, he's simply a politician. When Seldon starts discussing the end of the Galactic Empire, Chen puts himself in opposition to Seldon's plan. Why? Because he works for the Galactic Empire, and the end of civilization would be bad for business. Simple as that.
Okay, now Wienis is a war pig and the closest Foundation gets to the freeze ray antagonist. The Foundation has political power and technological advancements, and Wienis wants both, and he wants to get it one way: war. Hardin is against the idea of war with Foundation (go figure), so Wienis and he are in conflict, hence: protagonist and antagonist.
Wow. When you put their names together like that, they just sound like a bad afternoon cartoon show. But they're actually the antagonists of "The Merchant Princes," a.k.a. the guys in conflict with Mallow. These guys try to frame Mallow as a traitor to weaken the Traders on Foundation.
Why? Because the Traders, whom Mallow represents, have become too powerful, and Manilo and Sutt don't want the competition. Unfortunately for them, Mallow's a smart cookie, and Seldon's Plan doesn't seem to work in their favor. Sorry, fellas.