The episode title "Bad Code" isn't hard to figure this time, since Root already used the line last week ("The Contingency").
Where did Root come from? Childhood trauma? Sold her soul? Or is she just one of those who simply "want to watch the world burn" (to borrow a Nolan line from a certain movie franchise)? Well, you sort of find out. An event that seals the deal on her view of humanity is finally presented, but, as you might expect, there are still clues that come up even in the final moments (so remember your scene sequence and don't blink) that indicate this origin story is not that simple, and predates (and postdates) the incident. And we may never fully know any more than the fact that we don't know everything, which is part of Nolan's master storytelling--creating a complex, compelling villain whose motives you cannot fully know, and therefore who is that much more unnerving because of her unpredictability. (Once you've seen the episode, go back and review the scenes with the young Root as well as people's memories of her, and you'll see what I mean about her origin predating and postdating the incident. Awesome casting for both the adult and child Root!)
But Root isn't the point here. Nor is the developing plotline with the few remaining NSA and OSC people who know about the Machine. Nor is Finch and his efforts to somehow resist and interfere with Root. No, the point here is in that line I quoted in the title of this review. When you hear it, you will cheer. And you will get the point.
P.S. To paraphrase one of Reese's lines from this storyline, "Nolan you sly dog, you." Jonathan Nolan in interviews at Comic Con indicated that Root would be around for multiple episodes this season. He didn't say anything about them being sequential. :)