The man sitting in front of Mary Ellen O’Toole was, she says, a well-mannered guy. “He was low-key. He was nice. He didn’t swear.” He was very proud of his work, which he described in polite, pleasant tones.

His name was Gary Ridgway. His other name was the Green River Killer. His work was killing at least 49 women in Washington state throughout the 1980s and 1990s. He did it all while maintaining marriages, parenting and church-going, and he seemed very much the word neighbors often use to describe men who turn out to have headless torsos in their freezers. Which is to say, he seemed very, very nice.

The niceness paradox. O’Toole worked as a profiler for the FBI for 30 years, headquartered in Quantico. She interviewed the Unabomber. She worked on the Polly Klaas abduction, the Red Lake school shooting and the investigation of David Parker Ray — the Toy-Box Killer who tortured women in a high-tech homemade dungeon. What she found was that the most dangerous criminals were often the ones who came across as the most harmless. That’s how they were able to continue harming people.

“Over the years, I used to hear this all the time.” Other investigators would explain to her why they had disregarded suspects: “I just looked at him, man to man, and I could tell” that he was a good guy.

“Really?” she scoffs. “Really? That’s what you did? I was so put off by this mystical concept” of infallible gut instinct. “It’s a cop-out.”

It bothered her enough that she decided to write a book whose premise goes against everything humans want to believe about their hunches. Your gut instinct? It is wrong.

Dangerous Instincts,” released two weeks ago, takes anecdotes from O’Toole’s serial killer investigations and exports them to suburbia, reading like a mash-up between a self-help manual and a Thomas Harris novel. What can O’Toole’s experiences with the Baton Rouge Serial Killer teach you about analyzing the effectiveness of your decision-making? What can Phillip Garrido, the man who held Jaycee Dugard captive for nearly two decades, teach you about which sleepover invites your children should accept? Is it possible to tell whether the lawn guy is a psychopath, or just overcharging you on fertilizer? The lambs are screaming, and they are in your cul de sac.

“If there’s a strange, dark figure in your yard, that’s an easy one,” O’Toole says. “You’re calling the police.” But boogeymen are rarely so neatly packaged. People put themselves in physical or emotional danger in dozens of less obvious ways every day, from sussing out an online dating profile to hiring a financial planner.

Reading the book is likely to do one of two things. If you tend to be lackadaisical about things such as door-locking, then the book will introduce you to the deadbolt. If you’re already vigilant, then it will make you purchase a Navy SEAL dog with bionic teeth.

Several years ago, security expert Gavin de Becker found success with “The Gift of Fear,” a book-clubby selection that told readers to be afraid of everything their Spidey Sense told them to be afraid of. “Dangerous Instincts” goes beyond: Those things that didn’t trigger your Spidey alarms? Be afraid of them, too. Abandoning gut instinct is, in itself, a terrifying concept. Isn’t it the very thing that kept our ancestors from eating poisonous plants and petting saber-tooths?

O’Toole, who retired in 2009, lives in Stafford, Va., in a country-cozy home decorated with year-round Christmas trees and comfy bric-a-brac. She is the opposite of what one would expect a serial killer expert to look like, which, if you have read her book, means she’s probably exactly what one should expect.

“They’re low-fat!” she says, carrying a platter of cinnamon rolls onto her porch on a recent Friday morning.

She was raised in an FBI family — her dad was an agent and her mother was an occasional personal assistant to J. Edgar Hoover himself. O’Toole studied psychology in college and grad school; she thought she’d go into marriage counseling, though the humdrum domestic problems of her clients bored her to tears.

Then, while still a student, she took a part-time job as a J.C. Penney floorwalker to make some extra cash, roaming the department store on alert for shoplifters. One day, she watched a man covertly pick up piece of jewelry. And swallow it. Worried that the only evidence had now gone internal, O’Toole coaxed the thief into the back room, where security searched his duffel bag for other stolen goods. There were no other stolen goods. There was, however, a giant butcher knife. And a serial killer was on the loose in the area.

O’Toole was told to stay with friends while they investigated the shoplifter, who ultimately turned out to be guilty of nothing more than petty thievery. But the ordeal lighted up parts of her brain that the bickering couples counseling hadn’t. “This isn’t scary,” she thought. “This is exciting.”

In the past decade or so, criminal profiling has become something of a public commodity. Americans raised on a combo of Fear Porn and Me Porn now believe that casual “Criminal Minds” viewing has made them into psychological experts. (The handyman did it!)

“It’s an unintentional arrogance,” O’Toole says of the amateur gumshoes who ask her opinion, then disregard it and proceed to regale her with their own baseless theories. It’s the assumption that people who conform to societal norms — houses, kids, cars, clothes — must actually be “normal.”

It’s reassuring to think you live in a world that can be visually compartmentalized, where escaping danger is as simple as crossing the street to get away from the tattooed human pin cushion. It’s scarier when the danger is what awaits you on the safe side of the road.