I think everyone has an internal moral compass - “this is right, that is wrong”.
However, some people’s moral compass can go up and down depending on the circumstances - “if they do it, it’s wrong, if I do it, well I have reasons, that make it ok”. Then further down the line “it’s definitely Ok, because …….” and because they know the difference between right and wrong, even when they are doing shit stuff they can still justify to themselves they are behaving ok.
Thats why sometimes we can know someone for years and then be astounded at something we think is dishonest, but they don’t.
A dividing line might be where you buy something in a big store, the cashier undercharges you, (not your fault), and you don’t realise the mistake until after you get home, or even a while later, and then it will be a big faff to take it back, and quite likely if you ring them they won’t want it back (say cost less than a fiver).
So, you technically took unpaid goods out of the store (wrong), but do you morally feel you behaved wrongly. I think most people wouldn’t do anything, because it will only get sorted with action, and in a situation like this, inaction is just easier.
How else do scammers, thieves, fraudsters live with themselves?
Obviously, you also have people with personality disorders, and non normal brains eg narcissists, psychopaths, etc etc.
Our brains don’t develop fully until we are 25. So in the case of younger women perhaps working with an older senior (married)colleague, their sense of right and wrong can be influenced/distorted by said married man.
So in some cases the OW will only understand what she has done with further life experience and will be full of regrets, either because she was manipulated and lied to, or because she now can now understand the wife’s side. Sadly that tends to happen when young women get repeatedly let down by men.
Or she’s got no moral compass, is easily influenced and will never care or examine her own honesty.