*Obvious Spoilers*
Before I say anything, I'll say that I liked the episode. A lot of people are down on it for being a "Cliffhanger", but i don't really see it as a cliffhanger at all, so I liked it.
-summary-
In the episode BB encountered "Terra" who has mysteriously returned. He talks to her, but she doesn't "remember" ever meeting him or anything else TT related.
BB fights a Slade-bot, and the episode ends with "Terra" telling BB to leave her alone, and BB goes to help the other Titans.
-End Summary-
This is how I interpreted it.
The girl was not Terra at all. Terra's actual statue/body was destroyed or knocked over, from any number of things. This girl was just someone who coincidentally looked a lot like her.
The Slade robot wasn't "sent" by anyone. It was just a left-over Slade-bot from season 1 or 2 that was just acting on it's old programing. It had no really over-reaching plot or goal other than to antagonize BB.
The point of the girl, from an artistic and directorial, point of view was to illustrate to BB (and to us; the fans) that Terra was really gone. That we must accept her death.
So often the mentality of fans is "when will Terra be brought back?" or "Death is always temporary in comics/cartoons", and the point of this episode was to show that that's NOT what's going to happen. That we, and the Titans, have to grow up and mature. Which includes accepting things like people dying.
This is also interesting I find, since this is the last episode of the series. So the moral or the show parallels this, in that we must also accept that TT is over, just as we accept Terra's death.
Now, since this was the last episode, there's no "right answer" to what happened, which is why I called this my interpretation. So take the show's ending as you will, but this is what I personally took from it.