It’s actually not uncommon. Trauma and PTSD leave epigenetic changes in people. These can become hereditary through a combination of both nature and nurture. Unless treated, this leads to an intergenerational heightening of fight or flight responses and a host of other issues. This in turn predisposes people to do horrible things in the name of “survival” (in their minds), even when it’s not actually necessary.
In short, traumatized families are predisposed to inflict trauma unless treated.