Seventh Sister was a Jedi and a survivor of Order 66.

In Star Wars Rebels Twilight of the Apprentice, Ezra and Maul are fighting the Seventh Sister. Later, Maul restrained the Seventh Sister by dangling her in the air with Force-choke. Ezra was told by Maul to finish her off, but he couldn't. Then, Seventh Sister gave off a devious smirk, planning to use the advantage to kill him. Order 66 killed a hand full of Jedi. Some survived such as her. Also, Order 66 would have almost gotten her killed. She may have spared a trooper who was trying to kill her, and the trooper would use the advantage to kill her. Logically, she'd realize how such mercy would kill her in the long run. Think of it as if she was planning to project what she faced during Order 66 on Ezra.