As they become adults, they simply turn this relationship dynamic around and start acting out the "abuser" side of the relationship dynamic they have learned.
For example, someone with anger management issues, a diagnosis of intermittent explosive disorder, or a drinking or drug problem may easily get out of control during arguments (e.g., because there is something wrong with their ability to inhibit themselves at the brain level) and verbally or physically strike out at their partners and dependents.
Still other people who abuse end up abusing because they have an empathy deficit, either because of some sort of brain damage, or because they were so abused themselves as children that their innate empathic abilities never developed properly.
Such abusers cannot or will not relate to other people as people, choosing instead to treat them as objects. They treat people as though they were there solely for their convenience and do not otherwise have an independent, important life.
Understanding Abuse People have difficulty understanding the motives of people who are involved in abuse.
Why people choose to abuse other people is a common question.
Why (adult) people who are being abused choose to stay in abusive relationships is another. Their early history consisted of receiving abuse themselves and/or seeing others abused (one parent abusing the other or their sibling, etc.).
Neither of these questions have easy answers and even the strongest attempt to educate yourself as to why people might make these seemingly irrational choices will not lead to complete understanding. The first question, "Why do people abuse other people? As a consequence, abuse is the normal condition of life for these people.
Abuse situations must be lived in and experienced before their internal logic makes any sense. Such people internalized a particular relationship dynamic, namely the complementary roles of "abuser" and "victim".
They are familiar with and fully understand the terror of being the helpless victim from their own childhood experience.