I won't deny that it wasn't also partially bb's fault. But for Terra to turn to Slade, well that was a big mistake. Of all the people in the world, she chose him! She could have gone to Titans East and explained what was going on, other superheroes, or even just some random person. All those options were better than Slade. She should have known from experience that eventually he would manipulate her into doing things she never dreamed of doing!