But then Facebook's founder and chairman dived right into an extended discussion of the next Facebook frontier augmented reality, which integrates digital information with the user's experience in real time.
His mention of the killing, while seemingly sincere, still felt like a kiss-off.
But it's not surprising. Denial is, far too often, the Facebook way.
Remember just after the presidential election when Zuckerberg shrugged off the importance of the hyperpartisan lies in the form of news stories like Pope Francis supposedly endorsing Donald Trump?
"Personally I think the idea that fake news on Facebook, which is a very small amount of the content, influenced the election in any way I think is a pretty crazy idea."
In time, he changed his mind about that, and Facebook, to its credit, has made some significant moves to flag, limit and remove financial incentives for lies and misinformation that spread like a disastrous oil spill during the campaign.
But Facebook still hasn't come to terms with what it really is a media company where people get their news and which, especially because of the year-old Facebook Live, generates news content. Since it began, rape, a horrible attack on a disabled person, and more than one suicide has been live-streamed.
"The crux of this is what is Facebook's true nature: a technology that enables anyone to publish anything? Or a self-regulating media company with enforced standards?" Emily Bell, director of the Tow Center for digital journalism at Columbia University, told me Monday evening.
Facebook's answer became clear Tuesday morning.
With its nearly 2 billion monthly active users and more than $10 billion in annual profits, Facebook is better at making money and capturing eyeballs than at owning its equally huge power and responsibilities.
David Clinch, global news editor of the verification site Storyful, put it this way: "They have to take this issue very seriously and deal with it urgently, or they will surely face more calls for Facebook Live to be put on hold until far more robust controls are put in place."
So far, that's not happening.
In recent months, Facebook has gone out of its way to avoid acknowledging the obvious: It is a media company, not simply a platform its billion-plus users to share their lives with family and friends. (I called last year for the company to hire an executive editor, as one step, partly a symbolic one, in that direction; that was shortly after a Pulitzer Prize-winning photograph was deleted by Facebook, which saw its depiction of the famous Vietnamese napalm girl not as art but as child pornography.)
But there are, of course, business reasons not to accept that. As soon as Facebook acknowledges that it is a publisher and not a platform, it may open itself up to lawsuits that could cut into profits fast.
Better, the thinking apparently goes, to stress technological advances and the ability to connect the whole world with virtual reality, baby pictures and exploding watermelons.
And its Facebook Live has been a force for good, too. Last year, Diamond Reynolds live-streamed the police shooting in Minnesota of her boyfriend, Philando Castile. It was important piece of bearing witness, made poignant by the presence of Reynolds' tiny daughter.
At this crucial moment, Facebook's language often sounds clueless, with its combination of stilted corporate euphemism and childlike wonder about "community" and "sharing."
Following the Cleveland slaying, which remained on the site for hours, a Facebook statement put it like this: "This is a horrific crime and we do not allow this kind of content on Facebook." Later, a Facebook vice president made a seemingly more thoughtful effort to describe the ways the company would use artificial intelligence and a better "reporting flow" to address the problem.
But none of this was specific enough, or serious enough. Nor did Zuckerberg's brief mention help.
As Clinch wrote on Twitter: "There's no algorithm for this and there is no cheap way to do this with community monitoring and inexperienced staff."
Facebook is immensely and increasingly profitable it made more than $10 billion last year, up dramatically from 2015. More than four of every five dollars comes from mobile ads, which makes video a more and more essential to corporate success.
But this can't go on forever.
Bell summed it up: "If Facebook is really interested in the unbiased nature of discourse it would know that totally unmoderated systems favor the authoritarian bully, and suppress free speech rather than enable it. Ask Twitter."
An innocent man and his killer who committed suicide Tuesday are dead. But that can't be the end of the story.