His work is done. Zack's reaction is perfect. Especially if he starts to wonder why Alec isn't there to drive him off. Where is he, anyway?

Turning around to walk backwards, Death's Shadow said, "Bastard? Me? No. At least I know who my parents are." Then he decides to add, "I hope to see you again!"

Just to mess with Zack's head a little more. And he's off in the crowd, vanishing from sight.

Fighting in the Bar constitutes actually attacking someone. Which Zack did not and which is a good thing. He would not want to be stabbed or cut or just have blood touch that knife.