What sea animal eats whales?

Killer whales. Killer whales are the oceans most apex predators. They have been seen preying on whales including the blue whale.

The killer whale itself is actually not a whale it is a dolphin. The reason it is called a killer whale is because researchers and whalers saw the dolphin killing whales so they called it the killer of whales which has been over time shortened to killer whale. :)