What would you do if your friend attacked an innocent person? MY VIEW

The second most disturbing thing in the video is that the other three just walked away.

You may have seen it: Sunday, 2:15 a.m. in River North, a prosperous Chicago neighborhood full of restaurants, clubs, nightlife. A man is sitting on the sidewalk outside a condo building. He has vomited.

In the bright glow of streetlights, a female security guard comes out of the building rolling a yellow mop bucket. After she approaches the man, who appears to be in his 20s or 30s, he gets up, wanders a few feet over, props himself on a low pole.

She’s talking to him from a distance — the video has no audio but later she’ll say she’s telling him to move along — when he turns and hurls the contents of his water bottle at her. Then he punches her in the face. That’s right. He balls up his fist and aims it at a 46-year-old woman who has done him no harm, who has come to clean up his vomit.

The guard stumbles backward, bumps into the rolling yellow bucket, puts her hands on her cheeks, then staggers toward the door.

It’s appalling to watch, but the act of violence isn’t the only thing that makes the video disturbing.

There are three other people in the scene, apparently the man’s friends, a guy in a stylishly untucked shirt and two women in short skirts and high heels. They’re attractive young people dressed for a good time.

When the punch lands on the guard’s face, the women are looking in that direction. Their male companion turns his head and seems to glimpse the assault from the corner of his eye. And then they all stroll off. None of them goes over to the guard. Before they disappear from the gaze of the condo building video cameras, one of the women shoots the attacker a WTF look, but none of them says to the guard, “Are you OK? Can I help you? I’m so sorry.” Without even a backward glance, they just drift into the night.

And that’s disturbing.

What would you have done in their situation? Watching, I had to wonder that about myself.

A Chicago street at 2 a.m., especially one that’s home to a bar, as this one was, is not conducive to well-reasoned judgments. None of us can be sure — I mean really, truly sure — what we’d do if one of our friends assaulted an innocent person in similar circumstances.

But it’s hard to imagine any defensible version of what happened to Zoa Stigler that night or any proud defense for just walking away.

Stigler, who had been working at the building for a couple of months, wasn’t a physical threat to the man who punched her, or to his friends. She wasn’t another drunk on the street.

She was a woman doing her job, in the middle of the night when other people were out on the town.

Turning back to help her wouldn’t have been the equivalent of trying to break up a brawl.

“It almost makes me want to cry,” she said later in an interview with WGN-TV, talking about her experience of watching the video, “because I didn’t remember everything and to see him hitting me that hard …”

I’d guess that a lot of people who watched the video wanted to cry and wondered, as she did in the interview, why someone would punch her.

Because he was drunk? Because he has an anger problem? Because he holds women or black people in contempt?

Videos don’t allow us into other people’s hearts, so no one but the attacker can know the answer.

But it’s this other question that’s just as vexing: Why would you walk away if you’d just seen someone hurt that way?

Because you’re embarrassed? Disoriented? Because most of us, even as adults, carry a childlike fear of punishment and blame? Because you’d rather protect your guilty friend than help an innocent stranger?

Maybe eventually they’ll talk openly about it.

On Tuesday, a man was arrested in the attack after turning himself in.

“He knew what he did was wrong,” Dawn Valenti told reporters after accompanying him to the police station. She’s an advocate for victims of violence who has a friend who knows the man’s family. “Of course he’s scared. I can’t tell you how sorry he is. He’s sorry because of what he did to her. There are consequences he knows he has to face.”

In the meantime, Stigler has displayed impressive grace since the video was made public.

“I’m OK,” she said in a TV interview. “Things happen. I’m still alive.”

Maybe the rest of us can be reminded by this thing not to walk away from someone who needs help.