Had to take matters in my own hand in a similar situation. Gf's cousin of 8 was your typical brat who liked to punch and kick people. His parents never punished him for it. No consequences. And he kicked/punch HARD. One day I was chillen with him while watching tv. He decided to hit me in the arm for no reason other being hyper and unable to sit still. Had enough of his shit. Told him straight up, "I'm going to hit you as hard as you hit me so you can see how much it hurts." I'm pretty sure I hit him far softer but he still cried. I then said, "See. That's how much it hurts. Now you'll know what it feels like when you punch people." He cried for only a bit and just sat there. I really think he was thinking about what I said since the next time I was with him, he was chill. He even got along with his older sister, or at the least she wasn't screaming at him to stop whatever nonsense he usually is up to when he's not the center of attention. His little brother was the complete opposite of him. Sweet and chill and I was his favorite person outside of the family. So I couldn't 100% blame it on poor parenting. Some kids are just born assholes and need to be corrected.