I'd guess that fictional serial killers outnumber real serial killers by at least 100:1. After all, how many movies and t.v. shows are there about serial killers - and how many are there in real life? That's hardly surprising - serial killers are ideal dramatic villains. If they didn't exist, it would be necessary to invent them. And since they are virtually non-existent, it often is.
What's more surprising to me, though, is that fictional prison murders outnumber real prison murders by a similar ratio. As Steve Levitt explains:
In 2005, 56 prisoners were murdered. There are roughly 2 million inmates held in state prisons, meaning that the homicide rate per 100,000 prisoners last year was only 2.8. That number is less than half the rate of New York City (6.6 per 100,000) and an order of magnitude lower than Baltimore (42 per 100,000). Indeed, of the 66 largest cities in the United States, only El Paso, Tex. and Honolulu, Hawaii have lower homicide rates than the state prisons.
In short, the number of prison murders on the shows Prison Break (which I love) and Oz (which I don't) roughly adds up to the actual annual total for the nation.
What gives? Here's the voice of experience:
[P]risons are incredibly highly controlled environments. Whenever I have visited prisons, I have been amazed at how safe I felt. In contrast, when doing ride-alongs in police cars, I’ve always had the feeling that something crazy could happen at any moment.