Even if Ben wasn't operating under the knowledge that "what happened, happened", he could have wanted revenge against Sayid for killing him. What better way than to destroy the man's soul by using him as a cold-blooded killer? Ben has been fairly calculating with everyone from the Oceanic Tribe that has found themselves in the Dharma Initiative. If a resurrected young Ben ever learned what Sayid said during his drugged interrogation, it would have been enough to prompt his actions when Oceanic 815 crashed. (This would, in turn, explain why he seemed to know the plane was going to crash, and why he ordered lists to be made of the surviving passengers.)
As shocking as Ben's death was in this episode, it just doesn't make sense for it to "stick". For one thing, it's possible that he's only badly wounded, and others have recovered from similar situations (Locke, for example). It would also completely violate the rules of time travel that have been established thus far. (The only possible exception to the rule might be Desmond, but even he is only operating within a kind of "uncertainty"; the course still corrects over time.) One would expect that Ben's fate will begin to pull back the veil surrounding his relationship with Jacob.
Sayid's encounter with Ilana serves to clarify her motivations (if, in fact, she is being honest). It's entirely possible that she is what she says she is; it would fit into the general redemptive theme of the story if Sayid's past sins caught up to him in such a way that he had to return to the island. If Ilana is simply a bounty hunter, then that points to Caesar as a possible Widmore mole. (He, too, may have a more mundane motivation and background, but it never hurts to consider Widmore's possible moves in the game.)
There's little doubt that Sayid's actions will have a profound effect on the truce between Dharma and the Hostiles, especially if Horace decides to order some kind of retaliation. In turn, that should serve to complicate Sawyer's attempts to keep the ruse going. That said, there's a lot of time left in the season.
Overall, this episode returned to the original format of the series with the requisite fifth season twist, focusing on a character whose actions in the future informed his psychological state in the past. As complex as that sounds, the story was actually quite straightforward. The writers have continued to deliver with yet another strong entry for the season!