If she had, Gotham City would be rid of its deadliest criminal.
The second big Joker moment on this Suicide Squad trailer helps fill in the gap between when Harleen was literally shocked and when she accompanied Joker to Ace Chemicals to undergo her own transformation.
Joker, the insane agent of chaos that he is, is amused with her holding a gun to his head, even daring her to pull the trigger.
Alas, she hesitated, and Joker slaps the gun out of her hand.