Look at the way they protest. Look at these protesters march through an otherwise quiet library, yelling at people who just want to be left alone to study. Look at these BLM protesters shutting down a Bernie Sanders speech. Look at these protesters blocking off a street. Or these protesters, blocking a train. Narcissists never give any thought to how they inconvenience others, and it never seems to occur to them how they would feel if their lives were disrupted this way. Like children, they throw their tantrums without concern for the consequences.
(Look up "narcissism" on Wikipedia, and among the telltale signs listed are "difficulty with empathy" and "inability to view the world from the perspective of other people.")
The social justice warriors are so convinced of their own righteousness that they are oblivious to the fact that they are driving people away from their side of the argument.
(Wikipedia lists "a lack of psychological awareness" as another sign.)
If you've ever talked to a social justice warrior, you know they tend to be impervious to facts. Quote them a fact that doesn't fit with their narrative and they'll dismiss it with, "Oh…what's your source on that, Faux News?" If it is a conservative source, they'll often respond, airily, oh, I would never listen to anything they'd say. (This is a statement more revealing than they intend: narcissists only accept the version of events that fits with their narrative.)
Another narcissistic specialty is argument-by-drowning-out. I've never had a conversation with an SJW that hasn't ended with them trying to talk over me. Even if you listen to them politely, they will almost always interrupt and not allow you to have your say.
Anybody who refuses to even listen to the other side is a narcissist. This, of course, applies to people on both the Left and Right, but I've seen the behavior far more often from Lefties.
(Once again, "inability to view the world from the respective of other people." And, it's hard not to suspect, "problems in sustaining satisfying relationships," also per Wiki.)
Look at the difference in hate crimes hoaxes. They all seem to come from the Left, which needs to make up things to get angry about. When was the last time you heard of someone on the Right fabricating a crime in order to look like a victim?
(These hoaxes actually cross the line from narcissism to sociopathy, but since most SJW's don't commit them, we can't say such hoaxes are typical SJW behavior. On the other hand, it is interesting that these hoaxes all seem to be perpetrated by those on the Left.)
Consider the concept of "micro aggressions." It's a way to claim offense when none was meant. Traditionally, it meant saying something insensitive to black people like, "Can I touch your hair?" or "You people…." Now it can be just saying that America is the land of opportunity. Or "God bless you" after a sneeze.
(From Wiki: "hypersensitivity to any insults or imagined insults.")
By the way, when was the last time you heard a conservative complain about a microaggression aimed at him?
Being an SJW is, at heart, a pose. I actually had someone tell me once, "I'm a liberal. I care about the less fortunate." In other words, I'm a liberal because I'm a good person. Like that old bumper sticker advertising, "I brake for animals," it's all moralistic preening.
Some people become liberals because they've been brainwashed into thinking that they must be if they want to think of themselves as "good." And their desire to see themselves in that light is so strong, they can't even allow "bad" (i.e., realistic) thoughts to enter their head because doing so would sully their self-image. So, they put on, or at least pretend to put on, politically correct blinders.
(For some reason, some narcissists, like some sociopaths, have a strong desire to appear morally superior. It's not enough to be as good as other people: they must show that they are better.)
Contrast today's campus protests to the campus activism of the 60's. Social Justice Warriors today no longer protest needless wars (in Southeast Asia, or Afghanistan), or nuclear weapons, or for free speech. They now clamor for "safe spaces" -- where free speech is not allowed -- or against "microaggressions," those imaginary slights. Or against that nebulous (and mostly fictional) bogeyman, "white privilege."
It's no longer about larger issues: it's now all about them.
Their mindlessness and demands for ideological purity are reminiscent of Mao's Red Brigades back in the 60's. But instead of having Mao's Little Red Book, today's SJW's obey the (unwritten) Manual of Political Correctness, with its ever-evolving (and increasingly ridiculous) litany of required sensitivities and unmentionable facts.
But there actually is one group which beats the social justice warriors at their own game these days: ISIS. They are so intolerant of other's beliefs, so convinced of their own righteousness, that they give absolutely no thought to other peoples' rights and freedoms. Or even, to their lives. And they feel good about killing people in all sorts of grotesque ways, simply because they know they are right. Not a doubt in their minds.
If you view the terrorists' behavior through the prism of narcissism, and think of them as the ultimate social justice warriors, they become easier to understand.
See, that's the thing about being an SJW, or a narcissist: you can't ever harbor any self-doubt.