The ruthless killer is actually Vincent Sobel (Johann Urb). He was finally unmasked in tonight's episode, and Dinah was there when it happened.
To provide a little backstory, Sobel is Dinah's late boyfriend and former partner at the Central City Police Department. He was shot and killed by criminal Sean Sonus just as the particle accelerator exploded.
He was also the catalyst for Dinah leaving the force and becoming a vigilante herself. Of course that was before she joined Team Arrow as Black Canary.
ExtraTV.com was on the set of “Arrow” in Vancouver, where the cast opened up to reporters about the revelation.
Juliana Harkavy said it was a “shock” when she found out that Vigilante was tied to her character. She added that it complicates matters for Dinah, because even though she told the team about him, Dinah secretly allowed Vigilante to get away.
She explained, “Not only is she dealing with her past but she also has her own little secret that she has to decide whether she's going to keep from the team, so it definitely throws a wrench in her life and game plan.”
Echo Kellum, who plays Curtis, hinted, “I think you'll see some detective work on Team Arrow's part to try to get to the bottom of the truth.”
Stephen Amell aka Oliver Queen also weighed in saying, “I like the idea of another metahuman on the show. I think that we've handled having… two now with Katie and Juliana… I think we've handled that well and kept it within the spirit of our show. We'll see how it plays out.”
He also teased, “I've been doing some work with the Vigilante character just relatively recently.”