Im also beginning to see how damaging those early childhood experiences of one's ADHD being countered with punitive authoritarian methods and views. I can see it in my DH who is having to now learn how to parent our child without getting inappropriately angry at her for when she is actually NOT having a discipline issue, while also understanding when it IS appropriate to see it that way and how to handle THAT without anger. I now see why its been so difficult for him- he was never modeled any other way.
I understand you're really angry and understandably so. My DH is the one with ADHD but I'm the one that comes from a violent, abusive family. He, too, is outraged on my behalf about what I suffered. So while I get your anger, I don't think that abuse is an ADHD issue, it's an abusive family issue.
My DH wasn't diagnosed until after our oldest was (in K). He, too, has many of the challenges parenting that you mention - not because he didn't have appropriate parent role models but because of his ADHD. His parents were wonderful. He has poor emotional regulation and has difficulty 'editing'. He speaks before he thinks and parenting our kids, especially our ADHD kids is a challenge for him because he struggles with executive functioning.
I, on the other hand, had a horrific childhood and the model my parents presented is one I'm determined not to imitate. Yet, I'm a better parent than DH because I have excellent control, don't mirror my kids' emotions and am usually able to distinguish whether a behavior has it's roots in ADHD or something needing consequences/discipline. What I'm saying is that your DH's behavior and challenges aren't necessarily a result of his upbringing. I'm not trying to minimize the impact of what he suffered but much of what you describe could also have it's roots in his ADHD.
Growing up with untreated ADHD has it's own legacy. I understand your anger but the swath your painting with your ILs behavior is far too wide.