By being abused themselves.
Abusers became abusers by how they were raised. Spoiled and neglected children often become abusers because their parents conditioned them in both cases to reject accountability for their bad behavior. The parents who spoiled their children have kids that were conditioned to expect to get away with murder and not have to face consequences. They believe it is their right to do whatever they feel without suffering a consequence, even if it hurts others. They were raised to be self-centered, arrogant, always thinking they are right. And flying into an abusive rage when life drops little hints that the universe does not, in fact, revolve around them. A neglected child faces a similiar fate, although they will be less successful in life. Both the spoiled and the neglected never learned to tie bad behavior to conseqeunces, both never learned they needed to control their tantrums, both were conditioned into believing they aren't responsable for their behavior. Both were conditioned into this behavior by their parents (their lack of parenting, actually). The prerequisist for being an abuser is a profound selfishness and sense of entitlement. Abuse doesn't happen without those traits. Often they come upon these traits because they were spoiled growing up. Or their parents failed to raise them properly with discipline that fit their transgressions. A neglectful parent and a parent that spoils their kids can have the same effect. It creates a selfish, baby like personality that doesn't think it needs to face consequences, and throws a tantrum at the slightest discomfort. Its a myth to say that abused people become abusers themselves. Only self-centered, un-empathic, entitled people abuse. It is also a myth that abusers simply can't control their impluses. They can and do. Why else do they behave abusively behind closed doors, only to their loved ones...yet turn on the charm and act all sweetness and light in public or in front of authorities? They CHOOSE to abuse so that they can get away with it. They know what they are doing. They know it is wrong. They simply don't care. They are experts at behaving badly and getting away with it. They were raised perfectly for that.