system
Given the following post, respond as if you're commenting on Reddit:
human
Why do abusive parents abuse some of their children but not others?

I have my own observations based on anecdotal evidence, but I would like to hear what research has to say.
gpt
Bowen's family systems theory has some of the answer. Basically, you're born into a position that has an implicit script associated with it. Older brothers will always be older brothers, youngest sisters will always be youngest sisters. Abused kids grow up, seek out other people who they feel they can relate to, and clean up the abuse they received but still do a good amount of harm to their kids. 

Often, you see families set up a "good child" and "bad child" that follow from how their system is set up. Think of an overly authoritarian parent who has kids that respond by being pretty passive. These kids might discourage their own kids from being authoritarian, not seeing how they could be harming them because they're responding to their own pain instead of what is best for the kids.

Hurt people hurt people, and abuse can be on either side of a pole. Too much of one thing leads to too little of it. On mobile now, but Bowen's stuff is available for free online. Highly recommend.