Don't see it being Kate or Ana Lucia myself.
I'm still thinking Walt, but Sayid is a possibility. Out of the remaining 'originals'
Jack - No chance
Sawyer - Not likely in his state.
Sayid - Do they really want to portray an Arab man as a murderer?
Hurley - A dark horse, but it would require a big 360 and a lot of acting talent to pull this one off
Charlie - His eyes say yes. A strong bet
Claire - Post Natal Depression? Maybe Shannons a psycho and Claire retaliated
Sun - This would be a big shock. She's too meek to be a murderer
Michael - His temper could allow him to do it, but until the Walt issue gets resolved it's unlikely.
Locke - Anyone who's seen Stepfather will know Terry O'Quinn can play a chilling killer. It would be a shame if it was him
Jin - Can't see it
Walt - Who knows what the Others have done to him? Maybe they have harnessed all that weirdness that surrounds the boy and if Shannon accidently runs into him, she could be a goner. Remember when the Psychic gave a reading to Claire 'The child cannot be raised by an OTHER, rather than another'
Kate - Shannon could cross Kate and Kate has shown she will kill. I don't think that's the way the producers would go that way, but it would make for good TV
Boone - Don't think they'll go the Zombie angle, but maybe he's Freddy Krueger and gets to Shannon in her dreams. A longshot
Other 'originals'
Rose - No way
Scott/Steve (whichever one got killed), Whiny rash guy or any other main section survivors - No chance
Tail section
Mr Echo - No way. There is a deep character to be mined, no way do they make him a killer this early
Ana Lucia - It would be a disappointment, unless they played this for a couple of series. She also has unexplored depth
Bernard - No way
Libby - Not a major enough character and we know nothing about her (yet). Extremely unlikely
Cindy - We haven't even heard a word out of her. No way.
It won't be any of the Others, but 2 others could be possibles
Desmond - He would be the obvious easy way out. If the Producers want to cop out, it will be Desmond
Danielle - Unstable enough and also an easy option.
Maybe Shannon is pregnant with Sayids child and she doesn't want another baby to be born?