You are misunderstanding the acronym "SJW". It's ironic. It's designed to point out the absurdity of the behavior in question. It's about people who proclaim to be about social justice, but behave in ways completely contrary to that goal. By seeing themselves as warriors, they put aside the normal rules of civil discourse, and instead we get things like "there are no bad tactics, only bad targets". Lying and slander are considered perfectly acceptable, as long as they further the "goal". Being racist and sexist towards whites and men is perfectly fine to them, since they're the "enemy". We have incidents where they will simply make up rape allegations or post rape threats against themselves, because it furthers the "narrative". All of these warrior tactics, of course, undermine the very notion of "social justice". Wars are never just. A war may be fought out of necessity, but the war itself is never just. Those harmed in war rarely deserve it.
By labeling someone as "SJW", you are pointing out the intrinsic contradiction in their behavior.