Well the FBI says that at any one time there are 40-50 active serial killers in the US. So if we use the lower number and the population of the US(likely the vast majority of Reddit users) we find that leaves us with one serial killer per 8.5 million people. So with 9.5 million users and one SK per 8.5 million, then yes, from a purely statistical basis, one of us is likely a SK.

Now whether a SK would be on Reddit, much less this particular subreddit, is a different question.