She wasn't sure why she wasn't rushing to tell Antonin that Aubin was the one who murdered Hannah. Wasn't that just the sort of thing that Thorfinn needed to know? The massive wizard wouldn't rest until the monster who murdered his precious wife was in pieces. Something in the back of her mind told her to keep that secret to herself for the time being. Maybe it would work to her benefit later. William Wood confessed that the reason Aubin suspected Hannah was the Secret Keeper was because he'd been watching his cottage for days before they attacked them in Hogsmeade. When the only person that came and went from the cottage other than the Dolohovs was Hannah, he made an assumption. Aubin was cruel enough that he was willing to just assume that Hannah was the one he needed to kill. If she wasn't, he would've just moved on to the next suspect. The callous manner in which he murdered an innocent woman would've sent both Thorfinn and Antonin into terrifying fits of rage. She wanted to wait until the perfect moment to drop that bombshell. Mostly because she wanted to be present when he was eradicated from existence.