Are killer whales really killers; if so, what do they kill that makes them different from other whales without such a violent name?

Orcas are the largest predators of warm-blooded animals that we have ever known. Most whales eat either small fish or plankton. Fishes, squids, seals, sea lions, walruses, birds, sea turtles, otters, penguins, cetaceans (both mysticete and odontocete), polar bears, reptiles, and even a moose have been found in the stomachs of orcas. They even have been known to eat such things as blue whales (largest living species) and great white sharks.

For more info on their diet, see this link. For more information about Orcas in general, see this link.