The concept that we are all basically good people is the one that makes me sick
the question of inherent nature is something that has been studied by cognitive psychology for years and years and years, and very little that is absolutely conclusive has been established. a few things, however, we can be sure of:
[1] people are very subject to the influences of their surroundings.
[2] the difference between a very good person and a very bad person is much thinner than we imagine.
[3] knowing that we are very subject to the influences of our environment, it becomes more evident that early cognitive development periods have long-lasting effects on a person, calling into question any concept of pure bad
[4] this is perhaps the most important one: the conflict between man's biological drives--which are far older--with man's rational side. what a human being desires emotionally or the immediate visceral experience of being a human being is far different than the rational side of humanity. this is what i feel people are trying to explain when they discuss how good people can have bad thoughts or do bad things: they're trying to figure out why they can like their friend so much, but can still at times wish their friend ill. these desires are not inherently evil, and are often related to something much simpler, like the recognition of possible options (even those that are unsavory to the self), or to an animal desire to hoard resources. it is important for people to understand this split, and to mediate it.
[5] there is a distinct split between consciousness and mind, which causes similar conflicts. "personality" and the way we develop learned behavior is in many ways totally arbitrary. being able to separate your consciousness in you from what may be behavior that is not useful is one of the most important abilities of a human being. it's the ability to transcend yourself--to step outside yourself, look back at yourself when you're considering doing something you DON'T want, and to recognize it as such.
that's all to say i think the model you're applying to life completely flattens it into this handing yourself over to "sin"--those behaviors that produce undesirable human relationships--while explaining none of the reasons why you have those feelings. it backs a person into a corner and says, "you feel these things! and feeling those things makes you bad." but really, those feelings don't make you bad at all. it's the actualizing of those feelings that makes them bad: taking irrational jealousy (which everyone feels) and turning it into petty contempt and slander; taking personal insecurity (which everyone feels) and turning it into an affair.
doing bad things is bad, yes. it's bad because it produces bad results. that's the problem with the creation of a priori reasons not to do bad things: it doesn't tell you anything about the impact on your own human relationships. forming an actual understanding of inevitable effects on your interpersonal relations from an action makes the action more or less desirable. that's what ethics and morals are. ethics and morals stand in for pragmatic results of shitty actions.
so why should you not have an affair?
(a) sin. do not have an affair because it is a sin and because god wants you to love your wife.
(b) because having an affair creates unavoidable consequences in your relationship with your wife--even if she never learns of the affair--which make your life worse and turn you into a less happy person.
understanding B and understanding that your desire to have an affair comes from your awful, petty insecurity and not from some innate human desire to have affairs makes you far less likely to do things that will completely fuck up your life.
sorry for the long post, but i obviously have a lot of thoughts on the subject. i also apologize if i rambled here and there, but it's a big topic that is hard to grasp the scope of.