River kills Rory in the finale. She is already admitted murdering someone and Moffat said at one point he will kill a main character in S5.

River is Amy's mother? left her to be brough up?

Just an idea....

I've thought about River killing Rory too (think it's been mentioned elsewhere on here already somewhere). Although he'd have to do something pretty magnificent to become the greatest man she ever knew. I think realistically it'll be one of those long running character traits that we'll never truly get an answer to for a long time to come.

Amy's mother?! Personally I think it's highly unlikely as when they said goodbye to each other there was the barest of affection between them. Although I liked the way River said "See you Amy" - almost hints that they'll meet again. Almost...

Yes, but from Rivers perspective in Flesh & Stone she has already met Amy, as she says that the next time she will see the Doctor is at the opening of The Pandorica (so his future, her past). That might explain her lack of suprise at seeing Amy.

killing rory might make some sense.

he's the next most signficant male character in the series (at the moment) and so possibly the only other male character that it would matter river killing them (barring some major character introduced in the finale). the "magnificence" could be all be about rory's character development, maybe in the finale he ends up being more heroic than the doctor. plus it would get him out of the way so amy could go on travelling with the doctor without rory hanging around like a micky-smith bad smell.

that said, i'm also open to the idea that river killing someone is part of a bigger arc and may not be revealed/resolved this season. cue other wild theories: river kills an evil future incarnation of the doctor; river was the master's companion...