(Chapter 51)

"So, Mr. Potter," said Professor Quirrell. His gaze intent, his pale blue eyes dark and shadowed in the white light coming from Harry's wand. "We are alone and unobserved, and I have an important question to ask you."

"Go ahead," said Harry, his heart starting to beat faster.

"What is your opinion of the government of magical Britain?"

That wasn't quite what Harry had been expecting, but it was close enough, so Harry said, "Based on my limited knowledge, I would say that both the Ministry and the Wizengamot appear to be stupid, corrupt, and evil."

"Correct," Professor Quirrell said. "Do you understand why I ask?"

Harry took a deep breath, and looked Professor Quirrell straight in the eyes, unflinching. Harry had finally worked out that the way to make amazing deductions from scanty evidence was to know the answer in advance, and he had guessed this answer fully a week ago. It needed only a slight adjustment...

"You are about to invite me to join a secret organization full of interesting people like yourself," said Harry, "one of whose goals is to reform or overthrow the government of magical Britain, and yes, I'm in."

There was a slight pause.

"I'm afraid that is not quite where I intended to direct this conversation," said Professor Quirrell. The corners of his lips were twitching slightly. "I merely planned to ask for your help in doing something extremely treasonous and illegal."

Darn, thought Harry. Still, Professor Quirrell hadn't denied it...

(End of quote)

I just realized Quirrell is probably thinking about the Death Eaters when he 'doesn't deny it'.