I’ve been thinking it’s possible that Riddler isn’t the killer, but following behind the killer and leaving his clues on top of the crime scenes for his own reasons - Gas-Mask-Goggle Dude is the one killing people and leaving the “No More Lies” messages in red, while the Riddler cards and voice over stuff is Eddie’s. The two men could be making opposite points - with Riddler lording his initially greater knowledge over everyone.
If, say, the serial killer is acting out against people he thinks are part of the Court of Owls, or if we’ve got some kind of Tell-Tale situation where the Waynes are connected to a dark secret, then it’s possible that Riddler might know Bruce is Batman and this is connected to the killings, and perhaps even a future target - thus, instead of “your secret friend” being the usual serial killer creepiness, it might be an actual “helping hand” offered by a villain for his own delight.
Riddler as a Seven-style serial killer is a fun idea, but not as appealing to me as Riddler having solved a Seven-style serial killer case and intentionally letting the killings continue while making it clear he knows who it is as part of his own psychosis.