Eh. I think Snape already understood that Dumbledore was manipulating him. From the interlude with the confessor:

“It's strange," Snape said quietly. "I have had two mentors, over the course of my days. Both were extraordinarily perceptive, and neither one ever told me the things I wasn't seeing. It's clear enough why the first said nothing, but the second..." Snape's face tightened. "I suppose I would have to be naive, to ask why he stayed silent."