I think it's Lupin.
Imperius curse, traitor, I don't know what.
Something is wrong.
Something's been wrong all along.
Page 204, where Lupin "proves" that it's really him --
That's all common knowledge. Snape, at least, knows all of that.
Harry didn't really ask him anything.
The argument that made Lupin leave --
Something is wrong with this picture.

And what does the chapter title mean, anyway?
What bribe?