I don't understand this thinking.
If not for the murder what separates Rich from any other DNC staffer?
I think you question is what identifies him as the leaker, yes? There is evidence other than his murder, that associates him with Wikileaks. he leaked before he was murdered, that evidence has to exist.
The way you are viewing the case, if Rich's murder was in fact a random act of violence, then he couldn't have been the leaker. I find that to be logically nonsense.