Generally, killer whales will not eat polar bears, but because they can be opportunistic eaters, if a polar bear presents itself as food the whale may eat it. However, the two animals typically leave each other alone.
Due to global warming, which is causing the ice of the Arctic Sea to melt, polar bears and killer whales run into each other far more often than in years past. Both animals have to leave their native areas to find food. Because polar bears need ice to walk on, when the ice melts, it puts them in harm's way due to the fact that they are in open water.
If a polar bear is on an unsteady piece of ice, a killer whale may hit the ice block from below, causing the bear to fall into the water, making him easy prey.