One Friday morning outside the Westgate Shaw’s on outer Congress Street, a handful of people stood helplessly by as a young man kicked an old man to death.

It was back in 2001, and I was a reporter. I got there while the police were processing the scene. There was a sticky pool of blood on the asphalt, and some of the witnesses were still there, stunned.

One told me he hadn’t understood what he had been seeing until it was too late – the assailant had been so calm, the witness thought the man was kicking a box. Another said he initially thought the victim was a dummy, and by the time he mentally processed what was really happening, he figured it was too late to intervene.

Eventually, Brad Martin, a retired firefighter, came upon the scene and recognized what was happening. He ran past the onlookers and put his body between the young man and his victim, John McCann of Portland, who would die later that morning.

The young man, Derek Soucy, stopped attacking and explained what he was doing. The old man was the devil, Soucy told police. Just check his dental records, and you’ll see. He was found not criminally responsible by reason of insanity, and is committed to the Riverview Psychiatric Center.

It’s been 17 years, but that story has stayed with me. I can’t help but wonder how I would have acted. I suspect I would have frozen, with the other bystanders.

Last week was one of those bystander moments. I had known intellectually that people were suffering at the Mexican border. I knew that, going back to the Obama administration, unaccompanied minors were being detained. I knew that families in which people had different immigration status were being torn apart by deportations. I knew that people who have known no other home than this country were in danger of being shipped back to a place where they don’t belong because they have the wrong papers.

But the problem seemed complicated and the solution was, too, something called “comprehensive immigration reform,” which the people in Congress were supposed to figure out.

Then I saw a picture of a child crying while her mother was being searched by police. I saw pictures of children in cages, wrapped up in Mylar sheets for blankets. I heard their voices crying on a secretly recorded tape.

It didn’t seem complicated anymore. These are human beings who are suffering. I don’t want to be a bystander.

Back in 2001, I talked to Bill Thornton, a social psychologist at the University of Southern Maine, who explained what might have been going on with those witnesses at Shaw’s.

One thing that stops people from acting in an emergency, he said, is the feeling that they’re powerless. And people in a crowd will often look to each other for cues on how to act, especially when confronted with circumstances they have never experienced.

“They ask, ‘Is this an emergency?’ and they use other people to make sense of the situation,” Thornton said back then. “They may be looking at me and seeing that I’m not responding, so we end up misleading each other.”

The key character in the Westgate Shaw’s was Martin, the retired firefighter who pushed by the crowd and stopped the attack. And as soon as he did, other people knew what to do. Someone put a blanket on McCann. Someone helped Martin keep Soucy contained until the police arrived.

Once we recognize that we’re all human beings, it only takes a little imagination to put ourselves in their story. We all know what it means for a child to be taken away from her parents, or how bad things would have to be for us to take off in the night with our families, headed for another country. And if you’ve ever been afraid, think what it would be like to be afraid every minute of every day.

If there was one positive lesson from last week, it’s that we were reminded that things happen when people stop being bystanders.

They can change the world in ways that seem impossible as long as they believe they can make a difference. The writer Rebecca Solnit offered this advice for activists: “It’s always too soon to go home. And it’s always too soon to calculate effect.”

Sometimes all you need is one person who sees what’s happening and shows everyone else the way.

Sometimes it’s not too late.