The state takes actions under reasoned and measured approaches (barring obvious bad actors), whereas the private citizen/vigilante approach historically does not practice reason or restraint in their application of justice.
It's far too easy for activists and anti-activists alike to get caught up in the crusade, and it's not hard to imagine that someone names an "activist in need of a taste of their own medicine" that gets piled on before anyone realizes a mistake was made and an innocent was punished, perhaps even irreparably.