Why do we cleave to the presumption of Manichaeism rather than the Hammettian conception?* I suspect the answer lies more in psychology than in facts. We want to preserve what Richard Sennett called a purified identity and hunger for what Christopher Lasch called "psychic security". In an age of narcissism people want to believe - often without troubling to read a word of moral philosophy - that they are right and others wrong and the wish is the father to the thought.
There's a dismaying amount of truth to this view, and it's not something limited to politics, of course, but that's where it manifests most blatantly and painfully. Society is not in the business of training its people to be skeptical of their notions, of being comfortable with being uncomfortable, as Pema Chödrön might have put it. It's in the business of indoctrination, largely as a way to keep the whole machine running. That there are ways to do this that do not involve indoctrination is a point too subtle for most societies to grasp. After all, they're not in the business of putting themselves out of business and replacing themselves with something better.
We don't like thinking of ourselves, or as others, as fluid, because that makes it difficult to create hard-and-fast rule about life. We hate having to think on our feet. Even I hate it. One of the kids who made my life miserable in high school, mainly my being a jerk himself, has since gone on to enjoy what looks like remarkable success in a difficult and demanding field of work. The first time I found out about this, I felt a wave of annoyance: why couldn't he just conveniently remain that little putz forever, so I wouldn't have to change my mind about him?
But that kid no longer exists in this world, except as some image in my mind. Hanging onto that dead image hasn't done me any favors. I owe it to both of us to not let that block me from anything. (I still wonder what I'll say to him if I end up bumping into him at some conference, though. Ideally, I'll just be civil and leave it at that.)
We owe it to ourselves to not believe in absolutes for the sake of personal succor. But most of us do it anyway, unthinkingly and with only the most marginal of provocation, and often for nothing more than the sake of feeling good about ourselves — even if that good feeling is so short-lived there's no way to measure it except with a stopwatch. We like to think ourselves as pure, or at least purifiable, but that's not a synonym for good. If "good" only exists in our minds as the nature of the reaction we have to the outside world as it centers around us, then we don't really know anything about good, or evil, let alone what to do with them or about them.
Just to clarify: I do not mean by any of this that good and evil do not exist. I believe that most firmly. I also believe that our ideas about those things are not those things themselves, and from that stems the vast majority of our confused behavior in those realms. It depresses me that I should have to explain such a thing, but here we are.
Side note: I don't agree with the author's assertion that "nobody" watched Breaking Bad; I don't think the show would be selling as well as it is on DVD/BD if it were a non-starter. I do think, though, that too many people get entirely the wrong idea about what it's meant to be about, or how it's meant to be about it.