Suspect Jasper Spires; NoMa/Gallaudet Metro station; victim Kevin Joseph Sutherland. ((CLOCKWISE FROM TOP LEFT) Reuters, Evy Mages/For The Washington Post, Photo courtesy of Matt Grossman)

What should they have done? What would you have done?

There’s a young man being brutally slaughtered — punched, stabbed 30 or 40 times, stomped and repeatedly kicked in the head — in a Red Line Metro car, in the middle of a holiday afternoon, in the nation’s capital, in front of almost a dozen witnesses.

Yet no one on the train confronted the guy who was killing 24-year-old Kevin Joseph Sutherland right before the passengers’ eyes.

This tragedy has become a litmus test online, where the Internet is full of heroes and the heroic ways they claim they would have dealt with the attacker. Police charged 18-year-old Jasper Spires in the bloody Independence Day killing; they say he may have been high when he ­allegedly tried to rob Sutherland. And here’s one more detail to factor into your equation: Spires is described in a police affidavit as 5-foot-5 and 125 pounds. So, not exactly a huge guy.

Court documents have revealed that recent American University graduate Kevin Joseph Sutherland was stabbed 30 to 40 times during an attack on a Red Line Metro train July 4. The suspect in the attack, Jasper Spires, was arrested Monday and charged with first-degree murder while armed. (WUSA9)

There were 10 or so other passengers in car No. 3045, and the online second-guessers have been filled with derision for their lack of intervention. Critics have cited the so-called bystander affect — the bigger the crowd, the less likely it is that someone will intervene when a crime is being committed — and lamented what the entire episode says about our country. Never mind that the police always urge witnesses to avoid confronting armed criminals.

On Reddit, which bills itself as “the front page of the Internet” and where this was heavily debated, there were lots of brave, anonymous people.

“I’m almost always carrying a backpack with me. Assuming that I chose the ‘Fight’ response (which I think I would, I don’t have as much to lose as many people), I would use it to try to hit the assailant,” one overlord of the Reddit comment section said.

“As a large and much-more-experienced-to-violence than most male I’d probably try to help/defend myself. But I also carry a defensive keychain (Monkey fist),” another online ninja offered.

Even my babysitter, the formidable Miss Teresa, thought she would have sprung into action. “I would yell at him,” she boomed when she arrived at our house this week. “People in this country don’t yell at each other enough. I would’ve yelled at him. How could they just let that happen without saying anything?”

But folks did. A man who said he and his wife were on the train during the attack wrote a long and searing account of the brutal attack on Reddit — how his wife pushed her way into the next car to get the driver’s attention, and how he pressed the call button, only to get a faceful of knife from the attacker, who told him to “shut up” but didn’t hurt him.

The man said he cradled Sutherland’s head as he died. He and his wife have spent hours going through that afternoon over and over, wondering what they could have done differently. Police assured them that they acted properly.

Mayor Muriel Bowser plans to sign a bill Friday giving district authorities more power to take action against the selling of synthetic drugs which are illegal in D.C. (WUSA9)

Sure, it would have been risky to step in on a knife attack. There are plenty of mourners who wish their family members had not risked their lives to save others.

But it makes a lot of us uncomfortable to think we would have cowered instead of confronting Sutherland’s killer. We live in a country that celebrated the passengers on United Flight 93 who resisted Sept. 11, 2001, hijackers and may have saved hundreds of lives at the Capitol or the White House.

Did our 9/11 “Let’s roll” heroism get replaced by “Let it go”?

“Where are those people’s sense of humanity, compassion, responsibility for another human being?” District reader Jane Wholey demanded in an­­ ­­e-mail to my Post colleague Peter Hermann.

“This reminds me of the Lululemon murder, but is far worse on the part of the bystanders. When Jayna Murray was murdered by a co-worker [at] Lululemon, employees at the Apple Store next door heard the screams yet did nothing. Jayna suffered more than 300 blows and was fighting back and screaming during the attack. But nothing was made of the fact bystanders shrugged off any moral responsibility to investigate.”

People debated the idea that maybe we’ve all become petal-soft commuters untrained in real survival, combat and lifesaving skills.

“Enroll in Krav Maga [a self-defense system]. And become a volunteer EMT or take a basic combat lifesaver course,” admonished one rough-and-ready Internet grand master.

The gun guys were on this, of course. One armed civilian with one good shot could have saved this young man, they shot from the hip, ignoring the idea that the showdown was in a crowded train car with a dozen innocents as possible collateral damage — or hostages.

Okay, cowboys. How about we hear from someone who actually did take action?

Dylan Rawls, 31, was walking to his car in a Bethesda parking lot in May last year when he came across a brutal beating. He stepped in, stopped the fight and, police said, saved the victim’s life.

“I’ve gone over that night a million times in my head. I’ve thought: ‘That was really dumb. I wasn’t thinking,’ ” he said. “But that’s what happened. I wasn’t thinking. Had I stopped, thought about it, weighed the pros or cons, had I had time to react, I might’ve scared myself out of helping.”

Rawls never had combat training, no martial arts, “unless you count watching ‘Kung-Fu’ movies,” he said. He’s a state government worker who slays paperwork all day.

All week long, he has been thinking about those bystanders on the Metro. What would he have done?

He and his friends were running the scenarios. Could you stop the knife with a briefcase? Could you take the guy down?

Rawls is not sure. And he’s a little exasperated with the armchair heroes and their takedown of the witnesses.

“I didn’t want people to be so harsh,” he said. “You really don’t know unless you’re there.”