OnwardandUpward
It's surprising, when you mention estrangement and find out others have the same in their families. My old friend who I tentatively admitted it to wasn't surprised at all.
Nature over nurture, I suppose. Did you ever see the TV programme about when someone brought up some wolf cubs alongside some puppies? They were all given the same amount of love and everything they could need. By six or eight weeks the wolf cubs were home wrecking and a bit savage. The puppies, meanwhile were cuddly and cute, a bit bitey but generally nice. The program was meant to show that however you bring up a wolf they are not a dog.
We have kids, we love them and put them first but sometimes difficult genes from other family members come to the forefront when you wish they would not. We often talk about which facial feature a baby has or who they take after, but personalities and behaviours in my family seem to have been passed on, unfortunately. They say it skips a generation. Well, I would never behave like it.
It saddens me thinking of how much I loved them, how I cared for them so much and everything I did was all about them, but they can still turn on us, colossally betray us and act abusive without a shred of guilt or shame.
Yet I don't want their "duty" or falseness either. Sometimes I regret having kids except I have one who is kind and good! I wonder if this is a pattern that the abusive EC's kid will do to them. How would they feel? Of course they won't think it will ever happen to them. How could they because I never thought it would happen to me.
There was estrangement in the family before they existed (on my Mother's side)
The abusive head games have affected my health badly, so going forwards I am disengaging and thinking what's good and beneficial for me. I definitely understand why older adults talk about going "SKI-ing" (Spending Kids Inheritance) and doing what is best for me for once in my life. 
I identify with a lot of what you have said. I have 3 adult children. Two of them have the same father. One has been completely estranged from me for no good reason for 16 years. The other is low contact. I doted on them when they were children 🤷♀️ Like you, I wish I hadn’t had them, but I don’t spend time dwelling on that. I’m just aware that my life would have been easier. I grieved terribly for the one who completely cut me off for years. Every Christmas Day, Mother’s Day and birthday broke my heart. However hard I tried to ‘get a grip’, I usually took myself off alone for a few hours to cry. I also spent years trying to have more contact with the one who had very low contact with me. They only contacted/contact me when they wanted/want something.
After so long, I have come to terms with both situations. I quite like the low contact, having realised that the adult child in question can be very challenging for me in larger doses! Regarding the estranged one, I love them, but I don’t like them., so let them get on with it. Like you, I wonder if history may repeat itself with any of their offspring. Thankfully, I have a lovely daughter 😀