Surprised to see her again, aren't you? And on the heroes list no less! Well, that's actually what makes the story of Terra such a tragedy, she wasn't born a villain, or a traitor, she wanted to be a regular girl. But then she got powers, and she wanted to be able to control them. That was all.
The first few episodes she appeared in, Terra was indeed a hero. A bit uncontrollable powers wise, but a hero nonetheless. Plus, when the "Tournament Of Heroines" was made, Terra made the cut.
And in the end, she used the last of her strength to save the day and kill Slade in the process.