It’s payback for our 19th Century slaughter of their whale cousins.
Actually, killer whales get the name “killer” on account of what they do to their whale cousins. See The Last Vikings, if you can find it. I read it. It was a book about the decline of the viking way of life, and it has an account of a “grampus” whale chasing after a much larger, but terrified, whale and taking big chunks out of him. But AI has no record of it, and I don’t recall the author’s name.