I don't know who it is-- but the narrative is strongly hinting at Boomerang. Which doesn't make too much sense with the killer's abilities (apparently super strong) until you remember he had Miraclo in the Satellite. So here's the theory-- depressed about his son, and his confidence staggering, he goes on a Miraclo binge and decides to make the JLA pay. Mirror Master gimmicks (he has stolen and used them before) get him into the apartments, and Miraclo makes him strong (and manic). When we see him, he's on the downer after it's worn off. The only thing that doesn't make sense is the note at the end of #4-- unless he wrote it high also, just trying to upset folks. Oh, and Calculator could be giving him info. This issue establishes that they are friends, so for free. Oh yeah, he's in the Suicide Squad, so he knows Slipknot.
Or, he could be a red herring and the killer is somebody else.
