How do polar bears behave in Antarctica?

Actually, polar bears are Arctic animals, not Antarctic. The largest carnivores on the Antarctic continent are seals and penguins. These animals feed on fish and are quite approachable since there is almost nothing in Antarctica that would want to eat them. Antarctica is located at the South Pole; the polar bears you mention are at the North Pole. This is the Arctic Circle of Northern Alaska and Canada.
If your question was about polar bear behavior in the Arctic, it would be different. Polar bears hunt seals over breathing holes in ice. They also take advantage of seal breeding grounds as well as migratory fish. Polar bears enjoy carrion when they can get it. Polar bears can swim for miles in search of new hunting areas. If polar bears were transplanted to the Antarctic, they would be at the top of the food chain until they had eaten all of the penguins and seals. After this, the polar bear population on the continent would collapse without a food source.