The anti-abortion protesters in Buffalo, as elsewhere in the country, claimed to be nonviolent, invoking the example of the Rev. Dr. Martin Luther King Jr. and portraying themselves as defenders of innocent life. But their abrasive rhetoric and tactics sent a different message. Not infrequently, women arriving for appointments at Buffalo WomenServices and other clinics in Western New York were screamed at and harassed. The physicians who provided them with care were likewise verbally threatened and denounced as “killers” and “mass murderers,” sometimes at demonstrations in front of their homes.

It was a bullet, not those words, that ended Dr. Slepian’s life. But by repeatedly using rhetoric that demonized abortion providers as monstrous “baby killers,” the protesters increased the likelihood that someone in their ranks would eventually decide using lethal force to stop them was justified.

In recent years, a term has begun to circulate to capture this phenomenon — “stochastic terrorism,” in which mass communications, including social media, inspire random acts of violence that according to one description “are statistically predictable but individually unpredictable.” In other words, every act and actor is different, and no one knows by whom or where an act will happen — but it’s a good bet that something will.

When a drifter named Robert Lewis Dear opened fire on a Planned Parenthood clinic in Colorado Springs in 2015, killing three people and injuring nine, anti-abortion activists like David Daleiden described Mr. Dear as a “madman” acting on his own. A few months earlier, however, Mr. Daleiden had released a series of undercover videos that falsely suggested Planned Parenthood sold fetal organs for profit. The videos had a powerful effect, causing a surge of threats against doctors and abortion clinics and potentially explaining why the organization became a target of Mr. Dear’s wrath. (“Kill the babies, that’s what Planned Parenthood does,” he announced in court.)

Not surprisingly, Mr. Daleiden’s effort to disavow responsibility for the shooting in Colorado Springs rang hollow to Planned Parenthood, which knows all too well that incendiary language can spur extremists to act. Before he was murdered, Barnett Slepian knew this as well.