So...I've talked to everyone and I'm still no nearer finding out who the serial killer is. Any suggestions? :)
Do some more side quests for other people in the Dead City. There are some dependencies which are not immediately obvious. They may become obvious later, depending on your stats - either way, keep completing other Dead City side missions and you'll eventually find things will move forward.
You may guess the culprit before he's officially "revealed". You probably won't guess the whole story though, I definitely didn't expect it...
Without spoiling anything too much, just make sure that you are well and truly ready when the time comes. ;)