Slade seethed in anger and betrayal, he'd lost yet another young apprentice, Terra, the girl tossing him into boiling lava had almost taken his life, however, the lord of darkness himself had saved the mercenary.
He'd never believed the girl was ready for such an honor, he'd hoped that by choosing Terra it would get under Robin's skin. Oh yes, the blonde child he'd forced to work for him was merely a toy and his own personal weapon, the real prize was Robin.
When Terra returned with the R emblem and stated to have crushed the boy wonder to death, Slade saw red.
That stupid girl, how dare she kill the one person that had earned the right to fight along side him, Terra would pay for her uselessness. He'd loved when the girl screamed out in pain, her body convulsing on the earthy ground, her own suit punishing her faults.
Terra would learn, she'd better pray that his little Robin was still alive, for the boy was his, his to train, to mold into a killer, his to control.
Terra's continued screams were like music to his ears, she'd fallen unconscious a long time ago, but still yelled and writhed in pain as electricity courses through her body.
Robin never would have been so weak, his boy was far stronger than this bratty girl, Robin had managed to tear off half his mask in hand-to-hand combat, something Terra could never achieve, with or without those measly rocks.
The girl had no control over her powers, being driven by emotions will get you killed in his business, that's what made Robin perfect for the role, the boy was driven to complete the mission, no matter the risks.
Robin was so many levels ahead of his team, being trained by Batman, fighting villains daily while in Gotham and the short time he'd been Slade's apprentice had helped the boy far more than anyone realized, for it was with Slade's teachings that he knew Robin was still alive, his true apprentice would be coming to him very soon.
The girl screaming on the ground was the perfect test subject, he'd wanted to see the effects his new suit had on the human body, he manipulated Terra into trusting him, he'd turned her against her only friends, much like he'd attempted to do with Robin.
Robin and Terra couldn't be any more different and it was exactly these factors that drove Slade insane, he wanted Robin, not this pitiful girl begging for mercy, he'd seen what he could do with his new invention, with the suit he should have no trouble controlling his prize.
Everything had been for his apprentice, someday Robin would thank him for all that he'd done, they'd rule alongside one another, as if father and son "Soon Robin, very soon you will belong to me and I'll never let you go."
I always wondered what drove Slade to mess with Terra, he always has an ulterior motive and sense I love the rivalry between Slade and Robin I came up with my explanations :)