Well, I'd suspected for a long time he was a traitor. Ron Weasley, too.

Last winter, when I went home for hols, this notebook of his wound up in my trunk. Only it was spelled shut, warded, which seemed really suspicious. I got some help from Padma spelling it open, and took a look. Honestly I wouldn't have thought anything of it, if it hadn't been warded shut in the first place, but inside he had all these notes on plants and some of them said 'ask S.S.,' and there were a few other things...anyway, I wasn't sure what to make of it. So I talked with Mr Rosier, and he called up Bellatrix Lestrange, and she brought in Neville to ask him some questions, and it turned out he could occlude.

Because of course he could. All the traitors learned how, I guess, and that's how they managed to not get caught.

They didn't learn anything from him. What I heard, later, is that Hydra was supposed to practise her interrogation skills and she mucked it up and broke him so badly, he couldn't talk anymore. But you know, she must have done that on purpose, since she was a traitor, as well. To keep him from talking.

With mates like that, who even needs enemies?