Looking for deceptive John

[checkyagirl.livejournal.com]

I want a John (preferably BBC John) who is not what sherlock expects him to be.
Whether it be something unexpected in the past (assassin,secret service, incarceration, etc), something about him as a person (secretly a serial killer, sociopath,not human[mystical]), or his motives (sent by mycroft, there to kill and/or spy on sherlock for the govt.,etc.)
What I really want is a big reveal. Sherlock at some point finding out about John and what he decides to do about it.
Thanks in advance :-)
Looking for Slash, but gen is good too.