Once upon a time, and not that long ago actually, people understood that you needed some guardrails in place if you were going to do live TV. Hence the idea of the seven-second delay.

The evening news, the World Series, Saturday Night Live, the Jerry Lewis telethon — anything that went out “live” over the air was subject to a guy with a censor’s button waiting for a bad word or a blooper.

The reason was simple: stuff happens.

Then along came the brave new world of social media and out went the button with the bath water. Almost a century of accumulated wisdom about broadcasting got swept away practically overnight in the rush to give every slob with a smartphone his own personal PBS.

The hell with standards and practices. Information wants to be free!

“[Facebook] Live is like having a TV camera in your pocket. Anyone with a phone now has the power to broadcast to anyone in the world,” wrote Facebook CEO Mark Zuckerberg when he introduced the service in 2016. “When you interact live, you feel connected in a more personal way. This is a big shift in how we communicate, and it’s going to create new opportunities for people to come together.”

Lovely sentiment. The problem was nobody in Menlo Park thought through the consequences of a world without a seven-second delay. Utopians don’t do consequences. The road to hell is paved with new opportunities for people to come together.

The company that loves to move fast and break things is once again taking extreme heat, this time for the New Zealand mosque murders. The 28 year-old Christchurch killer broadcast his racist rampage using Zuckerberg’s “more personal” communication tool. Video of the atrocities quickly jumped to YouTube, Twitter, and the other big social sites.

No delay. No producer standing by to pull the plug. Just lights, camera, mass murder.

Thanks, Zuck.

This sicko wasn’t the first to narrowcast a crime. The social media giants say they are working overtime to stop it from happening, but last week’s livestream suggests progress is slow. It’s funny: They’re pretty good at banning conservatives who commit thought crimes. Pretty inept when it comes to censoring mass murder.

Fair to say Silicon Valley’s disruptors didn’t intend for their toys to be used this way. They thought livestreaming from smartphones would be great for catching cops beating up black teenagers. What fun to watch Elizabeth Warren pretend to be a real person on Instagram: “I’m gonna get me a beer!”

The millennial dream went no further than tuning in to watch Alexandria Ocasio-Cortez make mac and cheese.

But for the rest of us the cult of disruption is turning into a nightmare. Zuckerberg’s hubris knows no bounds. He wants his magic tools to be used only for good.

Turns out that’s not how the world works. Somebody older and wiser could have told him, if only he’d stopped to ask.

Facebook thought it was breaking the backward habits of an old-fashioned, outmoded society, but what if all they broke were the guardrails?

Matthew Hennessey is the author of “Zero Hour for Gen X: How the Last Adult Generation Can Save America from Millennials” (Encounter 2018).