there is absolutely no way to justify this behavior

I am 90% certain my soon-to-be ex has ADHD.  Our daughter was diagnosed, years ago.  There is evidence of the anger and anxiety that seem to go along with ADHD, in him.  All the hints were there for him to be self-aware of his ADHD (i.e. hello?  your daughter has been diagnosed?) but his ego was too big for him to have any self-awareness.  He had an affair and now wants a divorce.  I've reached the boiling point because tonight, my other child is struggling with an issue regarding a new friend - who is supposed to come over for dinner - but my daughter is embarrassed that her dad had an affair, that her parents are getting divorced . . . regardless, she is trying to keep her dad happy and therefore considering an invitation for him to come to dinner.  She has the character and empathy that he completely lacks.

I don't care what he did to me; I can handle it.  But there is absolutely no excuse for his cowardly, selfish behavior and how it impacts his kids. I absolutely despise him for only thinking about himself.  He has always lied (although I didn't think he'd have an affair).  He has always flirted, to make himself feel good.  He has always put himself above all others - his wife, anyone else in the room, his kids.  

I absolutely cannot fathom how any human being can be so selfish, particularly in regard to his kids.  Does anyone else understand this, and is it related to ADHD or is it just pure selfishness?

I was aware that he had ADHD, for years.  I believed he was narcissistic, although he wasn't extreme enough to have NPD.  

I don't know if I can ever come to terms for understanding a selfishness that is so deep.