Lets take a step back to before 316...
Sayid is killing a LOT of people for Ben Linus, a lot of people. We are to assume they are most likely working for Widmore. Ben never reveals exactly who these people are and why they all need to be killed.
What we do note is Ben has Sayid do the killing - Ben never bloodies his hands directly.
No Illana suddenly shows up and flirts with Sayid, and in the end she gets the best of him and takes him prisoner, pretending to be (or perhaps really is) a Marshal of some sort...
So how does Illana know that Sayid is killing all of these people when she confronts him and tells him she is taking him to face justice for the murder of one specific person?
Did Widmore tip her off??? Obviously Widmore has been keeping tabs on all of the Oceanic 6 since he's got photo's and locations of all of them as he showed to Locke. So now we have to wonder - does Illana have a connection to Widmore and therefor since she was directly working for Jacob - is Widmore either working for Jacob or is there some double-agent stuff going on, hence the title of the next episode "Recon" -- Meaning Another Con???
Ben had me thinking right up to the end of Dr. Linus that he was playing stupid this whole time and really wasn't clueless to everything going on... but his change of heart and following of Illana makes me think he really doesn't know what has been happening and may no longer be of any significant role, expect one - to kill the MIB - more on that later...
Back to Sayid - he is the only one of the Losties (not counting Aaron) who had to be FORCED to come back to the Island... everyone else came of their own decision, even Hurley who refused to go --- he didn't have to be forced, Jacob approached him directly and asked him to come back. Yet Sayid had to be made to come back...
Now you have to ponder this again - Illana is working for Jacob, loves him very much and must be acting on Jacobs request to not only return to the Island with Brahm and the other bodyguards, but it appears she had to bring Sayid back - so was it not Widmore who tipped Illana off - but Jacob?!?! Could Sayid be Jacob's secret double-agent...
The Plot thickens...
Back to Ben --- Just a quick note: Sayid couldn't kill MIB - why? He is not a leader!!! Only a Leader can kill Jacob/MIB... Ben is now the defacto leader of the island again with Locke being dead, even if Widmore returns - is Ben still not that last active leader? Either way, it will take a Leader to kill MIB or at least stop him...