The 51-year-old made a name for himself as a one-man band of digital news. But his latest project has led him to collaborate with anyone who shares his outrage over the murder-a-day mayhem and his need to explore solutions.
Asked why he created it, he quotes Neil Young in the song "Ohio," about the 1970 Kent State shootings: "How can you run when you know?"
MacMillan has marshaled everything he's learned as a street photographer, combat shooter, journalism professor, and fellow in trauma studies to document the crisis.
He first started viewing gun violence as a public health issue last fall at a Ceasefire Chicago conference where an epidemiologist named Gary Slutkin said the news wasn't totally bleak because at least experts know what to do with an epidemic.
"The quarantining already exists," MacMillan said. "We are a pretty segregated city." The violence is typically contained to the same neighborhoods, which is good if you don't live there, degrading if you do.
Transmission of the menace can be disrupted, he said, with swift intervention to prevent retaliation. Young people need better alternatives and help seeing the lasting effects of their actions.
Changing minds is the toughest part, he says. "We've done it with cigarettes, drunk driving, unprotected sex. It can be done."
By example, he pointed me toward a video on his site that profiles the classes that Mothers in Charge, an antiviolence group, led at the Philadelphia Industrial Correctional Center, counseling prisoners in anger management and conflict resolution.
MacMillan and his small team of volunteers - they're paying for their work out of pocket - try to present the why that's often missing from daily crime coverage. MacMillan rails at how much of crime reporting, particularly on TV, is toxic, sensational. All heat, no light.
He mines social media to understand what's driving the crisis. Seeing that the phrase "inphillyyougetshot" was trending on Twitter last month, he culled what people were saying about the circumstances of the killings.
"Just because you're in Philly"
"For going to Overbrook High"
"If your ex sees you with your next"
"In the cross fire, so learn to duck"
"If you're doing everyone dirty"
"If you fight too good"
"Not joining a gang"
"For frying someone in ball"
"For being funny"
There was one more. "For having Dreams."
"That's the one that sucked the air out of my chest," he said.
An hour after we bid goodbye, the e-mails started coming. One of his interns had crunched some numbers that might make people pay attention to the price of violence, MacMillan thought.
Using a formula created by a RAND Corp. study, and figuring for medical care, lost productivity, police time, courts and prisons, the four homicides, 13 aggravated assaults and two armed robberies over the weekend in Philadelphia would likely cost more than $20 million.
A few minutes later, I heard from him again. While we were sipping iced coffee in Center City, he said, there was another shooting, about four miles away, on South Ruby Street in Southwest Philadelphia.
A young man had been struck three times in the head. No suspect, no motive. The victim was clinging to life.
"When," McMillan asked, "will it end?"
Contact Daniel Rubin at 215-854-5917 , firstname.lastname@example.org or on Twitter @danielrubin.