He feels himself wither a little at the question. Though Mark assuredly doesn't mean it to be accusatory, the horror at least is evident. And perhaps he should be, though what Mark doesn't know is that while Isaac isn't dead by his hand, he had already killed once before then. When he'd rescued Mildmay, he'd done it by incinerating Malkar. But at the very least it hadn't been just for his own sake.

"Worse, maybe, but it's not--" he stops himself from saying 'not the point', because he doesn't want to sound like doesn't at all care about what he did, though he would not say he wished it hadn't happened.

"I ordered Mildmay to do it, via the binding-by-forms. But even the binding-by-forms isn't complete control. It's a push...a strong one, granted...in a certain direction...that can be resisted for a time. And he fought it then."

He ran a hand over his face and looked up at last, waiting for the judgement he expected as he continued with the last of it. "The Protectorate Guard found us then. I wanted him to suffer, the way he'd make Gideon suffer, and I had no time to consider it. So I did it. No, I didn't kill him. I sent him mad instead."