Given the way her father operated, Tony's supposition of Mystique's motives was entirely plausible, but Wanda realized that if Mystique actually wanted to make a new life for herself apart from the Brotherhood she need only shift into a new form and never return to the Genosha. That, coupled with her clear distaste for the American government, made it all too likely that she really was spying in S.H.I.E.L.D. Even if she were only passing on information to save her own skin, that didn't justify any of the atrocities Magneto might commit using the information Mystique gave him.
"And if I don't tell Nick, and a lot of innocent people die when my father attacks that would be because of you, too," she pointed out, reaching out to turn his face to hers and make him look at her again. After a moment she dropped her hand and her head, sighing. "But it would also be because of me and I've got enough blood on my hands to last a lifetime. I'm sorry, Tony.
"Mystique isn't stupid," she continued ruefully, "She wouldn't have survived so long if she were. Do you honestly think she'd go back to S.H.I.E.L.D. now and give them the opportunity to put her in front of a firing squad? All she's got to do to save herself from S.H.I.E.L.D. or my father is put on a new face. She doesn't need you to protect her. But Nick needs to know so he can be prepared to protect his people if Magneto does attack."
It was frustrating that he was so adamantly deceiving himself, but Wanda understood why he was making excuses. Tony was no fool, he'd see reason eventually. In the meantime Wanda could do her best to protect him from himself. "No one needs to know that you knew. I don't intend to tell Nick, anyway."