Imagine the events of Chapter 10 from Casta's perspective — the person who was at least partially responsible for the death of your niece suddenly shows up with said niece in her arms, barely breathing. Three revelations go through your mind — Glimmer didn't die when you thought she did, she's probably been alive for five years and you never got to contact her, and Catra might've just finished the job and taken away the last opportunity you had to see her. No wonder she reacted the way she did.
It's a good thing Catra didn't leave the Mystic Mountains any sooner. If she had, Drakon wouldn't be able to verbally (and immediately) warn the group that Catra was still on their Earth, so by the time everyone found Catra, it would most likely be too late to save Etheria.
