WS here, and I apologize ahead of time if I repeat anything already said as I haven't had a chance to read ALL of the responses just yet.
Hell, my wife was one of those people who never missed class in school -- because she knew she was 'supposed' to be in class. She is by the book kind and caring, an incredible mother who never missed a beat when raising our sons. Her work crews would lay down in traffic if she asked them, because she actually cares for them and enforces a safe working environment for them - a rare thing in her industry. The list goes on and on. So, yeah, her poor choices were a huge surprise.
OldWounds made a very good point here. Being compromised or deficient in one area does not necessarily indicate an entire lack of ethics or morals. For example, being a cheater is still a far cry from being a bank robber or murderer for example.
I don't mean to get too far off track here, but I should also mention that morality is very much a paradigm as opposed to a something with a solid definition. If you ask the people of a warring country about the "enemy", they will tell you that the enemy has no morals and kills without guilt, so when they return fire, shoot and kill the enemy it is simply for the moral reasons of protecting their families, freedoms and homeland. Ask the other warring country the same question and you'll likely get the same answer. Each feels that the other lacks all morality, while insisting that their morality is intact. This makes morality a shaky subject since each person defines morality based on their own experiences, beliefs, and terms.
I can really only speak from my own experience here, but I'm not sure if morality is the problem. I think most adults have a basic sense of what is right and wrong, and in most cases, will adhere to those beliefs. So it is not that they don't "have" morals, it is that, for some reason, they have chosen to step outside of those morals.
For example, if we were talking about willpower as opposed to morality, we could look at a person on a diet who sneaks in a donut now and then, and judge that person as having no willpower. At the same time however, that same person could probably be trusted to not grab your wallet off the table, not molest an attractive person in the street, or maybe simply to not eat YOUR donuts. A lack of willpower in one area does not constitute a lack of willpower altogether. I believe it is this way with morality as well.
In my case, my sense of willpower and morality were counter-balanced by a broken system of learned poor coping skills and fear-based responses. In the abusive environment that I grew up in, doing the right thing often resulted in a punitive or negative result, whereas bad behavior was often rewarded and praised. It is not that I didn't know the difference between right and wrong, but when things got really bad, I defaulted back to my learned coping skills and reacted in ways that protected and empowered me as a child, but in my adult life, were abhorrent and damaging.
I believe that most WS's are broken somehow. There is something in their background or makeup that permitted or even encouraged them to step outside of the lines of their fidelity and act out in ways that defy who they are and what they value. While this brokenness will be unique to each person, it is imperative for them to discover the origins and roots of the problem, or it will continue to haunt their lives and relationships over time.
As the betrayed spouse, the difference might not matter. The end result is that the wayward spouse betrayed and hurt you in ways you probably never even imagined you could be hurt. As such, you are not required nor even expected to understand or fix your wayward spouse. Lack of morality or a broken person? Does it matter? That is a decision that only you can make. All I can suggest to you is that it might help to make that decision with as much information as possible.
For example, a child may grow up picking their nose and eating it. It's gross to be sure, but the child doesn't really know any better, and it becomes a "normal", a way to manually and orally soothe themselves when stressed. At some point, an adult may point this out to the child, and suggest that they stop doing that, and so the child does so and over time gradually comes to understand why it is gross and to not do it. Under periods of stress however (as an adult) the person may go back to what they know, their "normal", and seek to manually or orally soothe themselves. Such a person might over-eat, or get massages, or even subconsciously pick their nose. While most people wouldn't look at an over-eater and think, "Hey, that makes total sense because that person was a nose picker as a kid", having that information helps to make a lot of sense from an otherwise hard to explain behavior. Stress occurs, the urge to soothe themselves is overwhelming, and at some point, the sub-conscience begins to poke at the amygdala (fight or flight) and the nose-picker starts to make excuses and justifications that allow them to soothe themselves by over-eating. Do they know eating that way is bad for them? Yes. Do they know it may shorten their life? Yes. Do they know it will make them look and feel crappy? Yes. Do they know it may even affect their marriage, or cause their kids pain? Yes. But the urge to do it overrides everything, and they end up telling themselves that it's not that bad, or they deserve it, or they can stop anytime they want, or that they simply don't care. Call it what you will, but the person has lost their self, and probably doesn't even understand why they are doing what they are doing, until it is too late, and maybe not even then. In the absence of a larger stressor, or in the absence of the knowledge of what drives their disorder, they may not be able to see, understand or stop their own, damaging behavior.
This is not to say they aren't liable for the bad behavior. A child molester may be simply repeating the abuse that was done to him as a child, and for that he has my utmost sympathy. But that doesn't mean I'm going to let him anywhere near my child. In a similar way, a wayward is responsible for their actions even if those actions are a result of their brokenness. THAT, in my opinion, is where the morality really steps in. Perhaps the lack of morality is not in committing the crime, it is in not owning it? (At least, once it is understood.)
For what it is worth, you might consider this... Let's assume for a moment that all of a wayward spouse's bad behavior (the affair, an addiction, a temper, trouble sharing feelings) is the direct result of some broken wiring from their childhood, and that through IC this broken wiring can be identified and repaired, good as new. Theoretically, the person would not only be "the person you married" again, but since the broken part(s) were fixed, they may actually be a better person than the person you married! Just food for thought.