Kill Them All
"I am well aware of anything that might threaten my life," the Witch muttered.
From the moment she met with Rex alongside Edward when confronting the Executor earlier, she could feel that there was this heat of animosity that Rex exudes. It was hidden, Rex tried to hide it—but its presence was too clear to not garner the Witch's attention.
Of course, the Witch's role in the fight was not done smoothly, there were hiccups.
She knew that there were some things that she could've done better.
But nevertheless, the Witch had a legitimate reason for going about her decisions earlier.
Most of the time, she feared that the Fifthborn might suspect her.
It goes without saying that the grand plan to take down the Executor took priority—and that was what the Witch exactly prioritized. Keeping the Fifthborn's attention away from her was the best way to maintain the clarity of the grand plan.