One is neither required nor encouraged to kill Ebora. She was intended to be a recurring character. We would have seen her again in MM10.One is required to kill the CW, but as in Yanir's case, not by direct assault.
You are partially correct in the case of the Desert Terror, though. Killing the Desert Terror is only possible for players who valiantly (or perhaps foolheartedly) rush in to attack. This requires either mindless hack-and-slash or thoughtful investigation so that one can replay the scenario to get the XP.