Terrence didn't really think of himself as a spy at all. He wasn't really trying to make himself useful to the revolution by providing information on specific plans of the Dark Lord or his Death Eaters; not only could that easily make it obvious that there was a mole, but that wasn't what he wanted to do. For the most part, he didn't mind the things that he was asked to do as a Death Eater, and didn't want to see most of those plans foiled. Especially because, so long as little plans were working out, the situation seemed less futile and the Dark Lord (hopefully) worried less about a coming revolution. The more surprise they had on their side, the better.
It wasn't about an ethical objection to violence or world domination, or even a dislike of the Dark Lord and what he stood for. It was about getting rid of the potion and the plague of mindlessness it caused. It was about not being persecuted and attacked by people who were angry about the Dark Lord. All of his reasons were incredibly selfish, but that didn't really matter, in the end. It wasn't like he had ever pretended to anyone that he was some kind of righteous champion of all that was good and fluffy and sparkly and right.
He cracked a grin at the last comment. "Could always write it anyway," he said. "Got to keep a sense of humor about all this, after all." He looked down at his drink, still untouched. He hadn't really been thinking about it, but he'd unconsciously decided not to drink it. Not that he thought there was a high chance of anything being slipped into it, but he'd only really wanted it because ordering coffee was what you did in a place like this. He kept stirring, even though the sugar had dissolved, the coffee now mostly forgotten.