Kevin Joseph Sutherland, 24, was stabbed in front of fellow passengers on a Metro train July 4. (Family photo courtesy of Matt Grossman)

Jasper Spires, 18, has been charged with first-degree murder in connection with Sutherland’s death. (Reuters)

I can’t stop thinking about that brutal July 4 slaying aboard a Metro train.

It was the first homicide on a Metro car, it happened in front of horrified passengers and it raises hard, haunting questions.

Kevin Joseph Sutherland, 24, was on his way to meet friends when someone grabbed his cellphone, punched him and stabbed him dozens of times. Witnesses say that Sutherland cried out as he was assaulted. That the assailant threw Sutherland’s phone at his unconscious body, then went on to rob others before jumping off the Red Line train.

After a two-day manhunt, a suspect, Jasper Spires, 18, was arrested Monday and has been charged with first-degree murder while armed, and that news came as a relief. But for days, I’ve wondered over and over: What would have happened if a few of the 10 or so people nearby had intervened?

And here are the deeper considerations: What are our obligations to one another in a civilized society? Am I my brother’s keeper? Should I be? Spires is not a large man. Court documents describe him as a 5-foot-5, 125-pound black man. Did unconscious bias make the teenager seem bigger or make his odds seem better than the nearly dozen people around him?

I was not there. I cannot imagine the speed of the attack (it took place in minutes between Metro stations), or how shocked and afraid I might have been at the sight of a knife-wielding man. There are also reports that he may have been on drugs.

A witness who was robbed told The Washington Post this week that she and the other passengers felt it was too dangerous to intervene. “I think we were all trying to stay away from him considering he had a knife,” she said. They didn’t know what to do, and no one was going to try to stop the suspect. It is an understandable reaction.

“We don’t want to second-guess in any way the actions of those who witnessed this incredibly violent crime,” says Morgan Dye, a spokeswoman for Metro. A statement from the agency read: “As a general matter, Metro Transit Police do not advise people to intervene or confront suspects, out of concern for their safety.” Instead they should notify police, note the person’s description and remain on the scene. Witnesses did this.

But I still keep turning the attack over in my mind.

If we see an abandoned baby, as happened when a woman left her 6-week-old by the side of the road last week in Anne Arundel County, we rescue it, right? And if, instead of being attacked, Sutherland had fallen onto the tracks, we can imagine a crowd rushing to save him. But what do we do if an intervention involves greater personal risk?

Is there even a way to ask these questions without judgment? And is the risk of judgment the worst thing that happens when these questions go unconsidered?

I don’t have answers. I just know I was reminded of the 1964 case in Queens where numerous witnesses (the number is disputed) saw Kitty Genovese stabbed to death outside an apartment and didn’t intervene.

I just know that my 17-year-old daughter recently completed a 14-week program at the U.S. Holocaust Memorial Museum that focused in part on the role of bystanders, and we now have a museum magnet on our refrigerator that reads, “What I do matters.”

If it were me, my loved ones, or any of us — because after all, we’ve all ridden Metro — I would desperately hope somebody would help. I would hope that’s what I would do. I cannot stomach that a man died perhaps realizing that help wasn’t coming.

I can’t stop asking myself what I would have done, because it’s not a rhetorical question.

I was walking through a park in April when I saw a couple fighting. At one point, the young woman lunged at the guy. He caught her and threatened to throw her into a deep pond. Others saw this as well, but the couple ended up in my path and I jumped between them. The man raised his fist.

“Don’t hit her. You can’t hit her,” I begged him. The woman reached around me and slapped him in the face. I grabbed her as she was still swinging and urged her to her car. Then I went back to look for my glasses, which had flown off my head. For hours afterward, I was trembly. I don’t know what I would have done if either had had a weapon.

It’s a scary world. There are absolutely things in it that can kill you. But there are also things we can’t live with. And even at personal risk it’s worthwhile as citizens, and a society, to ask what those are. We are reminded, again, in horrifying fashion, that these aren’t just rhetorical questions.

For more by O’Neal, visit wapo.st/lonnae.