Usually, they've done it before with little or no consequence, or have observed others who use violence as effective control, and learn that doing it again may result in the same. They often need anger management, or have some sort of mental disturbance that goes unchecked. Victims will often reach out for help multiple times. Here in the states, unless it was caught on video or there was a credible witness to the incident, many prosecutors will plea out to a lesser misdemeanor like criminal mischief or drop the case entirely if they aren't sure that a jury will be convinced. Many domestic abusers know this and will game the courts (both men and women). A pity that woman was failed so horribly.

10 years in prison for murder? Slap on the wrist for assault? I thought American justice was lax! Where is the incentive not to kill your partner when you’re pissed at them? Couple of years you can go back out and find a new punching bag!