This was her first and best earth friend for one year. Artemis made her feel accepted as a normal person without discrimination. She thought that aqualad took that away from her. That's why she did that. If anything, I'm surprised that you guys aren't mad at Dick for shrouding it in all that secrecy. Because as smart as he is, he should have known that something like this could have happened. Especially with Megan who is the only one who has telepathy. Just put yourself in her shoes. She thought that aqualad took something important for her. If anything, she could have killed him if she wanted.