From what information I have garnered, the Butei Killer arc is only a part of a larger Sherlock Holmes arc. My personal conjecture is that the real enemy is Sherlock Holmes or his direct child (Gen 2), and that the bigger picture of the hysteria mode seems to be some kind of evolutive ability of humankind.
Can't wait for the anime to animate the Shirayuki vs Aria chapter. It'd be gold.