Agree that it's not deliberate in most cases, but still destructive. Words hurt, and if said by a parent hurt more.
I was involved with a solo mum for some years ( leaving with almost nothing and having to start over from scratch ), and repeated all the bad things that had happened to me with her kids. Like you say, it was all I knew.
I did not even realise what I was doing till years later. The irony is that I would have made a terrible father in the years I'd most likely have had kids, but after that time past I learned enough to have been a good parent.