The opinions expressed below are Jon Keller's, not those of WBZ, CBS News or Paramount Global.  

Police say the man who shot and killed four people in a New York City skyscraper spent days traveling across the country to reach his target. Murders such as these are increasingly common as the United States becomes desensitized to mass shootings.

Traveling to kill  

Investigators say Shane Tamura, who lived in Las Vegas, went through Colorado, Nebraska and Iowa before arriving in New Jersey yesterday afternoon. Just a few hours later, he was in midtown Manhattan with an assault rifle.

Those details evoked memories of Luigi Mangione's journey from Georgia to New York last December, where he allegedly assassinated the CEO of United Health Care. In both cases, the men had nurtured grievances, Mangione with the health care system, Tamura with the NFL over its past reactions to brain injuries among football players.

"Social media has certainly had an impact," said Northeastern University criminology Professor James Alan Fox. "If someone has a grudge, a grievance, they can find other people online who will support them, who confirm their ideology."

"Desensitized towards homicide"  

The vocal public backing for Mangione is a sign of the times, said Fox. "In years gone by, we didn't have that. Oftentimes, perpetrators lived alone; they didn't have a lot of friends, and they were alone with their thoughts."

But now, grievance murders are, if not often embraced, easier to commit. "It's far easier, nowadays, given the laws for people to carry concealed weapons and sometimes open carry," said Fox. "We still have a problem that individuals who shouldn't have guns are able to get them. We are desensitized towards homicide in this country."

No wonder. Footage of near-daily mass shootings at schools, churches and bowling alleys from Maine to California pours into our living rooms and laptops.

Increase in shootings as payback

Cases like the Mangione murder or the Manhattan office slaughter stand out because they had a specific target of the killer's grievance, while others, like the killing of those four Idaho college students, are terrifyingly random.

The sick work of a sick mind? Yes. But: "The percentage of mass killers that are schizophrenic, seriously mentally ill, is only about 10%," said Fox. "The rest are unhappy with their lives, believe that they've been victims of injustice, and they want some payback."

What can be done to prevent incidents like this? According to Fox, not much, although it might help if the political establishment heeded the pleas of police organizations to ban assault-style rifles like the one the Manhattan killer was wielding and expanded red-flag laws that keep guns away from disturbed people.

But in the longer term, Fox said, we need a cultural change away from wallowing in and even glorifying violent crime. Social media platforms, conventional media outlets, and other pop culture portals can be part of the solution just as they've been part of the problem.