I've spent time - years - trying to understand why my parents were as they were and did what they did.
There were aspects of my father's childhood that were difficult. On the other hand his sister shared a lot of those childhood difficulties, but turned out to be a warm and loving parent. It is entirely possible that my father was not neuro-typical, and had Aspergers Syndrome. However I know other people with AS, who are gentle and do not resort to violence against others.
I think my mother's values and emotions may have been systematically eroded when she got married to somebody who was highly manipulative and controlling. On the other hand some people who are married to partners of that kind do ultimately leave, for the sake of the children. Or seek to make amends to their children - belatedly -after the controlling partner has died. My mother did neither of these things.
I think their belief of theirs that they 'did their best' is/was deluded, based on a entirely mistaken set of ideas about what parents were entitled to do and about what children require in order to flourish.
It was like being brought up in a sort of mini-cult, because nothing could impact on their own sense of their righteousness. So I suppose my parents 'did their best', in the sense that religious fanatics may believe that they are doing their best.
I could choose to say, 'My parents? Oh they did their best.'
But that would be like subscribing to their delusion, giving it a validity it absolutely does not deserve.