I was pretty sure it was that the doom was because people weren't choosing their own justice, but following along with someone else's idea of what's right because it's what's easiest and most convenient.
Picking something and sticking to your guns is difficult, and often ends up leaving you ostracized, or sometimes just learning you've been wrong for a while about things. It's so much easier to rally behind someone else, because then even if it doesn't pan out, it's not your problem.
Each of the phantom thieves has their own way of going about coming to terms with their desire to help others being fueled by really selfish means. Their forms of justice are based on their own background, and not one single way of looking about it would be healthy for society at large. Hell, other than the fact that Shido was a condescending dick, the only thing that made him in the wrong was that he was forcing his ideology onto others in a unilateral way- what he had planned may have been in the interest of everyone overall, but the fact that he would force it onto others without others having a say ultimately was the problem.
And that's representative of a larger problem with how we treat justice, not just how we look at examples of it in history or media, but how it pans out in reality. The first step to achieving a just society is to remove any blockages from equal opportunity. From that point, everything else has to do with a shifting equilibrium of social value.
Some people debate where the equilibrium should fall, and it has a lot to do with representation of populations, classifications of peoples, and those kinds of things. Some argue that forcing it one way or the other through token representation and arbitrary classification leads to more injustice. These complex issues are what creates the shitflinging between "sjws" and people who vehemently hate them.
I don't feel I can really come to a conclusion about it, other than everyone should have opportunity.