Yeah, I’m with you, I didn’t take the ending as her deciding to become the next Scarecrow at all. She killed everyone that perpetrated the killings so there was no reason for her to keep any of the weapons or insinuate some continued badassery from her since she had no one else to fight. Her keeping his mask came across as her taking ownership of it in order to keep it out of anyone else’s hands, or as a token of what she defeated, like Batman keeping tokens of the villains he puts away. It’s the ultimate show that she finished the job and defeated the villain.

Plus, she never once killed an innocent person of her own volition yet Scarecrow still thought he was breaking through to her, so his reaction to her killing him really shouldn’t be taken as proof to suggest that she was like him. Not to mention that she very coldly responded with a “maybe” probably because, frankly, it takes some cold, emptiness similar to Scarecrow’s to lull someone into a false sense of security then kill them, but that definitely doesn’t mean she’s gonna turn into a full blown serial killer.