This penultimate episode of the season (and perhaps, series) continues the process of wiping out John Connor's support system, while bringing him ever closer to a confrontation with Catherine Weaver and whatever she has up her sleeve. As usual, even when the action is mounting, the emphasis remains on character.
I still believe that Weaver will turn out to be the leader of a faction among the machines that opposed the genocidal plans of SkyNet. Weaver wanted the Turk, I imagine, to co-opt the basis of SkyNet and create, in John Henry, an equally powerful AI with human ethics. Why this would translate into SkyNet (either the future version or the "current" version) sending a terminator to take out Savannah is not quite so clear.
It could be a matter of SkyNet waging a kind of war with John Henry, trying to take something that John Henry values. If SkyNet wants to weaken and subsume a potential threat, perhaps gaining something in the process, then stripping away John Henry's support system is a logical part of the equation. Taking out John Henry also means eliminating competition for resources.
Pretty much the same thing has been happening for John, and losing Derek was just the most recent and most crushing example. Derek was John's touchstone to his father, and while he made a mistake with Jesse, he was still John's loyal soldier. He's dispatched so quickly and methodically in the middle of the operation to save Savannah that it's a lot more shocking than it would have been otherwise.
John's reaction is one reason why Riley had to go first, however. John's entire season arc has been about holding on to his innocence, even as he feels it slipping away with every passing moment. Looking at John now, one can see how hardened he has become. Considering where the character started, this is proof positive that they had a plan for his evolution.
The writers also pulled quite the coup by killing Derek in the first act, thus effectively masking the notion that anything else of huge importance would happen before the end of the episode. Sarah's arrest comes as a complete shock, and it definitely appears that Weaver set her up to eliminate a potential obstacle to her own plans. After all, Sarah doesn't know that John Henry isn't the same as SkyNet (assuming that supposition is true), so Weaver must assume that Sarah will do everything possible to derail her plans.
It all comes together in the next episode, which may turn out to be the series finale. The producers promised that the season arc would be resolved, even if the episode sets the stage for a third season. It seems foolish, with the new film only a couple of months away, to take this show off the air and fail to capitalize on renewed interest in the franchise. But FOX has been more patient than I thought they would be, given the ratings, and if this is the end, I'm thankful for the excellence that we did receive.