I don't know if you can really talk about 'bad' parents at all - I grew up in a pretty abusive environment and I have come to understand that the mistakes my parent made stemmed from their own legacy of pain from their childhood. Unfortunately they chose to not attempt to heal that pain in themself (because that involves going to some scary places) and so instead played out unconsciously onto the next generation. I think that's what happens a lot - but when you do the work on yourself - to heal your own brokenness, then you have half a hope of passing on something good and whole and worthwhile to your children/other significant relationships.

I have observed over time that parents generally try to do their best - and children often blame them for this, particularly once they are adults themselves. So I have a saying that I think really distils the most important parts of the relationhip: It is the parent's job to love the child UNCONDITIONALLY, and provided they have done this, it's the child's job to forgive the parent for being human.