This is not mentioned nearly enough. I think it’s rooted in the idea that people must be either great or awful when being both is a very real possibility.
Another possibility is that being a good or bad are not inherent properties of people -- but only properties of actions. Bojack Horseman explains it well.
> That's the thing. I don't think I believe in deep down. I kinda think that all you are is just the things that you do.
and
> There's no such thing as "bad guys" or "good guys." We're all just...guys, who do good stuff sometimes and bad stuff sometimes. And all we can do is try to do less bad stuff and more good stuff [...]
Fred Rogers was unquestionably a good guy. He still made mistakes, and was very upfront about this. He made mistakes from the bottom of his heart trying to do the right thing but not always having the information (or patience to gather and process) to make better decisions, but he absolutely always made decisions trying to, even when advancing his own interests, take others into account.
Unfortunately, there are people who make decisions always with the intent to hurt others. Many of the actions that they take are individually neutral or good. It's hard to get anything done if you don't do some cooperation in society. It's quite probable that even for the most awful people, if you count unweighted they've taken more good actions than bad - but the magnitude of their evil is much higher.
Most people are neither. Most people are stupid and selfish but trying not to do too much bad. Bojack gets this, but somewhat misses the other implication - that being good and bad is learned and practiced, and that you should learn from and practice the ways people who are Good at being Good people,
"I built bridges for 20 years and no one ever called me Joe the bridge builder, and I paved roads for 20 years and no one ever called me Joe the road paver. But I fucked one goat..."
Human judgement can be fickle and outright vain at times. If there's an idea of outright good or evil, I don't trust any human (let alone society) to cast a proper judgement as such.
"If only it were all so simple! If only there were evil people somewhere insidiously committing evil deeds, and it were necessary only to separate them from the rest of us and destroy them. But the line dividing good and evil cuts through the heart of every human being."
It's "deep down" but not some unchanging, inherent aspect of "you". You just need more work to tackle it, and probably with help, not alone.
But yes, "good" it "bad" absolutely doesn't work when evaluating a single individual life.