“Orcas were given the name ‘killer whale’ by ancient sailors’ observations of groups of orcas hunting and preying on larger whale species. They called orcas ballena asesina, or ‘killer whale.’ Their Latin name, Orcinus orca, also reflects this observation of orcas feeding on large whales. Orcinus translates to ‘of the kingdom of the dead,’ and orca refers to a kind of whale.”
I’ve never seen a convincing source for this factoid. As best I can tell, “killer whale” was never “whale killer” in recorded written text.
“Orcas were given the name ‘killer whale’ by ancient sailors’ observations of groups of orcas hunting and preying on larger whale species. They called orcas ballena asesina, or ‘killer whale.’ Their Latin name, Orcinus orca, also reflects this observation of orcas feeding on large whales. Orcinus translates to ‘of the kingdom of the dead,’ and orca refers to a kind of whale.”
Source: https://us.whales.org/whales-dolphins/facts-about-orcas/
So seems like this etymology makes more sense.