I'm sure it was Alfred - The Outsider. He was the main orchestrator of The Crime Syndicate's arrival and was setting everything in motion for the collapse of the Justice Leagues.
What I would like to know though is how his stealing the Kryptonite ring ended with it in Despero's possession... if that was The Outsider I have no idea how he contacted Despero or what his plan was in giving him the Ring. (EUH)
I thought it could be Deathstroke, because the guy looked like he was in shape. I wonder why it was not clearly stated who it was.