There are people who do get beaten or killed for who they are. Transgender people regularly fear for their physical safety. Minorities get abused and even killed by the police on a daily basis. So when people protest this sort of injustice online or in real life you'd expect the author to be supportive of this. Except no, she apparently considers this horrifying mob-like behavior of social justice warriors.
It's the lazy stance where she supports social justice activism in theory, but when it comes down to actually protesting abuses of power (which can include rioting and other mob-like behavior) she considers it unacceptable. Never mind that historically speaking large scale public protests (= mobs) have often brought meaningful social progress. I wonder if democracy can even function without it, because people must have a way to make their voices heard when the official channels are closed to them.
Don't get me wrong. Mobs are blunt instruments and without question scary. However, they're not uniquely or especially scary. So when I hear something like "[mobs] are the thing that frighten me most in the world" I have to roll my eyes. Other political issues are scarier by several orders of magnitude. Nuclear war. Global warming. NSA spying. Democracy getting hijacked. Perpetual war. Not on this list: Internet rage directed at Sacco.