I think it's more along the lines of, "WSs are people." Full stop.
Some, I'm sure, are horrible, selfish, entitled narcissists, perhaps even sociopaths.
If narcissism and sociopathy were the primary causes of infidelity, then the answer would be easy: divorce.
Given that the statistical incidence of infidelity far exceeds the statistical number of narcissists and sociopaths in the population (I'm riffing off of very general numbers here, obtained in any Google search) then it stands to reason that some infidelity is committed by normal, sane, decent people who do something really shitty.
Getting to the "why" in this case is the $64k question with which many (I'd venture that most) BSs and even most WSs struggle.
We've had countless threads that go on for many pages in which one camp espouses the position that affairs are all about illicit sex, while the other camp insists that the sex is a value added (or, perhaps even not) but that the primary driver is some other need/compelling desire. It is often divided between gender lines.
I was watching an Esther Perel interview/talk recently (I KNOW, I KNOW, Esther is NOT an SI favorite) and she said something I found incredibly enlightening and insightful. To paraphrase Perel:
In every relationship, there is one partner who is more in touch with their fears of abandonment, their fears of losing the other partner, and there is one partner who is more in touch with their fears of suffocation, with their fears of losing themselves.
According to Perel, the partner who is more in touch with the fear of losing himself, and who is more invested in maintaining that degree of autonomy and self-investment, is the one more likely to cheat.
It's not that the other partners, who are more afraid of abandonment, of losing the other partner, or, framed in a more positive manner, are more committed to the relationship than to self, it's not that they don't find others attractive, or that they do not fantasize about or even overtly desire sexual or romantic involvements with other people. It's that those partners will not give themselves permission, while the partner who is more self-invested will.
When viewed through this lens, it brings much of the "why?" into a different, and sharper, focus.
I've thought about starting a thread on this topic on SI, and I still may do so.
Bad person? Or just someone who, for a variety of reasons, is more self-invested than relationship invested? It isn't even necessarily a malicious reason... it could be FOO issues, it could be a simple maturity issue, it could be part of the inherent dynamic of the relationship. In fact, it could even be healthy in some circumstances- although the decision to give oneself permission to cheat is hardly healthy.
IMHO, cheating is a symptom. It's a weather report on the relationship, and a barometer on how that person is dealing with being in the relationship. Bad person in a good relationship? Good person in a bad relationship who chooses a shitty way to cope with it? Giving oneself permission to cheat has nothing to do with the relationship? Could be anything, I guess...