Zach chose the process of elimination to test all these theories on Grace so as to ascertain the true reason with the help of the polygraph despite Grace's continued silence throughout the interrogation.
"Judging on the basis of how you waited for such an opportune moment to take action against Mr. Windsor and Violet, you must have some heavy grudge against them, am I right?" Zach decided to confirm the last possibility first because the rest of the interrogation would be carried out to determine exactly when Grace's loyalties shifted to the Hunter family.
