The term 'killer' has a few senses: 1: the causal agent resulting in death; 2: predatory black-and-white toothed whale with large dorsal fin; common in cold seas; 3: someone who causes the death of...
It is actually spun around a bit to make a name, it's actually named after it being a whale killer, one of the very few creatures that can do so. It is not a whale, but rather a dolphin.