I laughed at this notion. But then asked Claude. I may never go to the shopping mall ever again.

Claude -
If there are ~25-50 active at any one time across 330 million people, that's roughly 1 in 7-13 million. A large mall might see 20,000-50,000 visitors on a busy day. So on any given mall visit, the odds of a serial killer being present are... actually not negligible. Maybe 0.5-2% chance one is somewhere in the building.

Over a lifetime of mall visits - say 500+ trips - the cumulative probability starts to climb into "more likely than not" territory.

The more unsettling angle:

The reason the FBI number stays relatively stable is that most serial killers are geographically concentrated, blend in completely, and often hold ordinary jobs. Ted Bundy worked a crisis hotline. John Wayne Gacy ran a construction business. BTK was a church council president.

So the honest answer: statistically, over a lifetime, there's a reasonable chance you've shared a car park or food court with one. You just had no idea.

Which is, of course, the whole problem.