This undated photo provided by WDBJ-TV, in Roanoke, Va., shows station reporter Alison Parker, left, and cameraman Adam Ward. Ward and Parker were fatally shot on air Wednesday, by a former staffer in Moneta, Va. Credit: WDBJ-TV via Associated Press

“You can’t unsee it.”

It’s a warning, but also a flirty dare. The phrase is often used online, jokingly, about spoilers for upcoming episodes of “Game of Thrones” or a particularly goofy meme, say one that merges the faces of celebrities to create one uber-ugly offspring.

But it’s also used to warn of real danger, like a well-worn sign hanging at the entrance of a dark Brothers Grimm forest. The gruesome autopsy photo, the disgusting viral sex video too notorious to mention here, the horrific escalator death in China captured on a security camera. Don’t Google them. You don’t want to know and you won’t unsee.

On Wednesday, it was video footage of the fatal shootings of two Virginia TV station employees, Alison Parker and Adam Ward, as they were broadcasting live on WDBJ-TV early that morning.

For a brief window, if you happened to be online and active on social media networks, footage flooded Twitter timelines and Facebook walls of the TV broadcast, some captured on cellphones by people rewinding their DVRs and uploading the shocking scene.

Then something much worse happened.

I was standing in line at a drug store, skimming Twitter, when a friend retweeted a video of the shooting with no words or context. I wondered why. Lots of people on my timeline were warning others not to pass it on for reasons of decency and to deny the shooter, who was still on the loose, extra attention.

I clicked and was directed to the profile page of what appeared to be a new and not-very-popular Twitter user. I clicked the video and with growing horror realized it was a first-person version of the shooting, apparently uploaded directly from the killer’s phone. The video called to mind action shooter video games and the recent trend of vertical-video streaming services such as Periscope. Inexorably, the bobbing gun in the frame pointed to Parker and fired.

I couldn’t unsee it.

This screen shot shows the Twitter page of Bryce Williams, whose real name is Vester Lee Flanagan II, shortly after he fatally shot WDBJ-TV cameraman Adam Ward and reporter Alison Parker during a live broadcast in Moneta, Va., early Wednesday morning. The station said Flanagan was also an employee at WDBJ and appeared on air as Bryce Williams. Credit: Twitter via AP

A few minutes later, all tweets on the profile and video disappeared. The account was suspended. A Facebook account apparently set up by the shooter was also yanked. Then, the killer apparently shot himself in a standoff with police.

In the real world, it unfolded in hours. Online, the damage was done in a few minutes.

For the rest of the day, as happens on social media, the usual debates flared. The gun control one started before the killer was even found. The media debate about whether it’s ethical to show the broadcast video, never mind the shooter’s version, raged. CNN decided to show the original video once per hour. WDBJ would not show it at all. Other media outlets, including the American-Statesman, posted stills, links or videos about the shooter’s video, but not the unedited footage itself.

On KUT-FM’s “Texas Standard” radio show, University of Texas at Austin School of Journalism associate director Wanda Cash said she would not have chosen to run the video. Cash, who as a journalist once had to help decide what images to publish after 9/11, said, “I’m not sure if there’s a compelling public interest here… This is a tragic situation that is a private grief for the families and friends and co-workers of these people. I’m not sure that we need to see this as a loop on TV.”

According to The Associated Press, the station said about 40,000 people saw the shooting live on television.

The way copied videos spread online, it’s impossible to tell how many have seen or will see the shooter’s POV video, but it’s safe to say it will live on the Internet for a very long time for anyone who seeks it out.

I wondered the rest of the day whether watching the video had been a mistake and if I would have chosen to had I known what I was going to be seeing. I wondered if it’s irresponsible, especially for journalists steeped in a culture of gun violence, to avoid witnessing what the real — as opposed to fictional or virtual — consequences of that culture look like. When we turn our eye away from evil, do we allow it to go on unchecked, making such acts sanitized and theoretical?

On social networks, where Twitter and Facebook allow videos to autoplay as you scroll past them, can we even avoid seeing what we don’t want to see when things like this happen? Will there be more, and should potentially disturbing videos be blocked or filtered somehow?

Online video has been instrumental in framing the conversation about deaths involving police and race, as was the case in a bystander-shot video of a South Carolina shooting that happened earlier this summer. This was a video that couldn’t be unseen, but that also needed to be seen.

It was a dark day and my stomach churned as I thought about how the media and web dwellers would have handled a situation like this had the Sandy Hook massacre’s shooter posted video online. Would we have drawn the line at videos of children being killed? Would social media’s crime watchers have been so eager to pass along that footage?

Several friends I follow on social media suggested that Wednesday’s act of online hijacking and murder were a “Black Mirror” moment. The British anthology TV series is a collection of one-off, near-future technological horror stories, and the Virginia shootings indeed evoked an episode named “The National Anthem,” in which an entire nation watches, stunned, as a terrorist inflicts upon the public live streaming video they can’t unsee.

I was shocked and horrified, but not surprised, at social media’s role on Wednesday. The shooter, disgruntled, fired, was trained in media, where every significant moment must be recorded, linked, streamed, tweeted and status-updated to get a discussion going.

Social media is not a master plan for attention, and there’s no indication the killer was some sort of online wizard with a grand, orchestrated plan. But it’s likely, like so many people online, there was the instinct, or at least the twitch of a habit, to use the online outlets built for, “What do you think?” and, “Look what I did.” A knowledge that if something is so horrible it can’t be unseen, it will most certainly be seen many times. And discussed. And shared.

An always-on culture has taken to live-streaming hours of video-game playing, nightly walks, sunsets, rambling conversations or, literally, paint drying on walls. It can’t be a surprise that a maniac with a gun won’t use the same tools to make their misguided, fatal points.

Social media is the mirror and the megaphone, reflecting and amplifying each triumph as well as each irrevocable exit.

It’s impossible to unsee, but it’s getting harder not to see it play out in the first place.