He said he’d been waiting to kill people for a long time,” 18-year-old Anastasia Boylan told “Good Morning America” from her hospital bed.

Chris Harper-Mercer, 26, murdered 10 people and wounded Ms. Boylan and six others at Umpqua Community College in Roseburg, Ore., Oct. 1.

Harper-Mercer asked his victims to identify their religion, she said. Those who said they were Christians he shot in the head.

All but two mass shootings since 1950 have taken place where ordinary citizens are forbidden to carry guns or are unlikely to have them, according to John Lott, president of the Crime Prevention Research Center.

In its survey of mass shootings between 1982 and 2013, Mother Jones magazine found whites and blacks were mass shooters in about the same proportion as their numbers in the general population, though media often portray mass shooters as almost exclusively white males.

Harper-Mercer’s father is white, his mother is black. The Los Angeles Times said he had “white supremacy leanings.”

In the wake of the shooting, President Barack Obama called for “common sense gun control,” citing as examples countries which essentially forbid private ownership of firearms. But no gun control measure he’s proposed would have stopped Harper-Mercer.

UCLA law professor Eugene Volokh found zero correlation between gun laws and homicides. A 2004 evaluation of existing research by the National Academy of Sciences failed to identify any gun control measure that had reduced violent crime. The NAS attributed this to a lack of data.

As gun ownership in the United States has risen, violent crime rates have fallen. Violent crime is rampant in cities with strict gun control laws. There is very little in Vermont, which has virtually no gun regulations. Writing in the Harvard Journal of Law and Public Policy in 2007, criminologists Don Kates and Gary Mauser found that in both Britain and the United States, violent crime rates are lowest where legal gun ownership is highest.

Norway, Finland, Germany, France and Denmark have high rates of gun ownership, but low murder rates, Messrs. Kates and Mauser found. The murder rate in Luxembourg, where ownership of handguns is banned, is nine times greater than in Germany.

There were 14,249 murders in the United States last year, according to the FBI’s Uniform Crime Report. That’s a rate of 5.2 per 100,000 people, tops among the advanced nations in the Organization for Economic Cooperation and Development. Ten cities, all with strict gun control laws, had 2,398 murders, a murder rate of 9.53. The murder rate in the rest of the United States was 2.71.

Mass killers may be insane, but they’re not stupid. They choose for their murder sprees places where they know they are unlikely to be interrupted by a law-abiding citizen with a gun.

“The suspect in the Charleston, S.C., shootings in June originally aimed to attack the College of Charleston,” Mr. Lott noted. “He chose a church instead because the college had armed guards.”

In his 141-page “manifesto,” Elliot Rodger, 22 -- who murdered six and wounded 14 near the University of California-Santa Barbara last year -- said he ruled out various targets where there might be armed security.

Some of the students who survived the Umpqua massacre want to get guns to protect themselves. In a Rasmussen poll in June, 68 percent of Americans said they prefer to live in a neighborhood where they and their neighbors are allowed to own guns. In a Gallup poll last November, 63 percent of respondents said they think having guns in their homes makes them safer.

Every mass shooting has ended when the killer was shot by a good guy with a gun, or committed suicide when good guys with guns arrived. These are usually police officers, but, as Mr. Lott has reported, “Case after case occurs where concealed handgun permit holders stop what would have been mass shootings.”