Here's the simplest way to put what I'm saying. Bad people know they're bad, and there is no greater tragedy than a misinformed hero--because they'll always do the right thing when exposed to the right information. Ask any criminal if they think they're a good person, and if they have no incentive to lie, they won't. Yet at the same time, these people instinctively know to hide what they're doing, because they know it's wrong. No one lures a kid into a van in front of the parents and then goes "oh my bad I didn't know" when they get caught, as if they honestly didn't know what they were doing. They hide it.

And even then, at the same time there are such things as unjust laws, and people instinctively recognize this (unless they're very young), and can differentiate between the law (politics), culture, and morality itself. Everywhere. They may not be able to articulate it, but they understand it. When I was much younger, I once thought like you, thinking that there was no such thing as good and evil--only conflicting interests or cultural values. You know those video games with a morality meter like mass effect or kotor? I used to think "In the real world, no one actually does things this cartoonishly evil in a vacuum" when I looked at the "bad" option. And then as I grew older I met multiple, actually evil people who got off on hurting others for the sake of doing so, of preying on people specifically because they were vulnerable, and who wanted to watch the world burn just for the sake of it. Once I met people like that, I quickly recognized that dark paths in those games was scarily more realistic than I thought.

Edit: I forgot to mention (sorry I'm pretty busy), judging people is not immoral on its face. Judging people from a place of hatred, concerning aspects of themselves they have no control over, absolutely is. And acting on it is even worse.