The Killer Key is for law enforcement officials, landlords with squatters or evicted tenants, and pranksters who thrive on taking things one step too far. See, the key blade inserts into any Kwikset or Schlage brand lock (get the Schlage here), and then breaks off at the bow with minimal lateral pressure. Once deployed, the Killer Key's lockout effects are permanent; no one can get in without drilling the lock. (But anyone on the other side of the door can still open it normally to get out.)
According to Killer Key manufacturer Shomer-Tec, about 90% of the locks used in the US are either Kwikset or Schlage, so if you've got beef with someone, legal or otherwise (but hopefully legal), and want to eliminate their access to what's behind door #1, chances are the Killer Key has your dirty work covered.