I think one of the signs of maturity is when you realise that your parents like everybody else on the planet are flawed individuals as a result of their upbringing and genetics.They had their own shit from their own parents to work out and get together and like all of us some of us are able to get our shit together, some of us are not and the rest of us are working on it.
Becoming a parent doesn't automatically make all your baggage go away, and often that's when the baggage comes home to roost and the decisions they made were them attempting to deal with their baggage in a way that wasn't the most healthy but seemed like the right thing to do at the time, or maybe, they just didn't know any better.
Also assigning today's knowledge and sensibilities to past generations is a bit unfair, 20/20 hindsight makes us all enlightened.
Hopefully each gen learns something from the mistakes of the past gen and does a little bit better but even the best of parents will leave some scars on their children and them in turn will leave some on their children.
Shit happens it's what you do with it that counts.