Wienis is the closest Foundation gets to a true, old-school space opera villain. How do we know he's a baddie?
Basically, this guy is one eye-patch and maniacal laugh away from being the leader of the Injustice League.
But what really makes this guy an antagonist in Foundation is his love of "direct action" and his sense of nationalism. Let's start with direct action:
Wienis says of himself, "I have always believed in direct action. I have believed in carving a straight path to my objective and following that path. I have accomplished much that way, and I fully expect to accomplish still more" (III.6.34).
Notice how this puts Wienis in opposition to a character like Hardin? Hardin chooses to innovate new paths, discover new ways to solve problems, and constantly question the old ways. Meanwhile, Wienis picks a path and runs with it, regardless.
Usually, this path of direct action is a matter of violence. Is someone in Wienis's way? Kill him. Does Wienis feel someone threatening his power? Murder the man. Does Wienis not like the cut of someone's jib? Shot him in the jib (read: face).
And violence leads to nationalism. When someone is nationalistic, Asimov argues, their blind devotion to means that they can't see anything bigger. The needs, wants, and values of others? Forget about it. In other words, my country is the best; all other countries, meh. And "meh" is Wienis being nice.
Wienis wants the Foundation's power for Anacreon, because it'll strengthen Anacreon and enrich himself. But compare this to Hardin's Foundation. The Foundation provides its technology services to the other countries. It enriches them while at the same time protecting itself. That doesn't sound very nationalistic to us. But Wienies sees something he wants and just takes it. Direct action. Wienis. Done.
Of course, this path toward direct action ultimately leads to Wienis's poetic death as the man shoots himself with his own blaster. So, let that be a lesson to all blaster-wielders out there.