This is what happens when the people who dominate the public discourse openly manifest hatred and contempt for our duly elected president-elect and, by extension, his supporters (they routinely call Trump supporters "fascists"--and can you say "basket of deplorables"?). They assume that anyone who even voted for Trump buys into every loopy thing he says or writes.
The enlightened, open-minded left--and let's be honest, have we ever seen activists so ill-informed about the issues?--has declared that Trump won't be their president. They say they will resist and obstruct and impeach him. They have no intention of giving him a fair chance--none--they want to destroy his presidency. Why? Because, they insist, he's incompetent and a bad person who hates anyone who isn't a white male.
Let's not kid ourselves that it's Trump they hate. To this day, most of them still hate Ronald Reagan, even after he won the Cold War without firing a shot. They hated the Bushes, John McCain, and Mitt Romney. The fact is, they don't like anyone they consider to be a traditional American--because they've convinced themselves that traditional Americans are racists, sexists, xenophobes, and homophobes even though they aren't. Their hate is irrational. It could aptly be called childish if it weren't downright pathological. Now, finally, they'll have a Republican president with well-known character flaws, which means they don't even have to hide their hate anymore.
And the Facebook torture incident is what happens. The culture of hate they've manufactured has given license to the sociopaths among us to act out. We can be certain we'll see more of this sort of thing.