The bunny avenger is a monster that the developer Nakano15 created on his mod. The Bunny Avenger has a 1% chance of spawning when killing a bunny. Since bunnies have no value when killed, it is best to ignore them unless farming for them. They also have a chance to drop the Bunny Avenger Shield.
Bunnies that die from other enemies or environmental hazards (eg. slimes or lava) do not count as attacks from the player.