Murders Around Seoul

In the calm city of Seoul, people roam around in the streets. Children are laughing, adults are having their wonderful freedom. Until one night, there was a scream; a bunch of screams heard in a quiet area. The next morning, police found at least 15 people dead. Since then, more murders have began appearing, none linked towards one another. They don't know the motive of the person committing these crimes, a lust for blood perhaps. They do know one thing though. Next to each crime scene, investigators always manage to find a small piece of paper.
There's a tiny bunny drawn on it.
And the words "stupid bunnies get what they deserve"