I have on a few different occassions. I once nearly beat a guy to death who had almost choked his girlfriend to death outside of a bar. She wound up with a crushed larynx, but survived. I jumped on him and pulled him off of her, and he turned on me. I beat the shit out of him, and me and two other guys held him until the police arrived and they took her, and later him, to the hospital.
I also once witnessed a domestic violence murder. One summer, about 30 years ago, I helped a friend build a massive deck in his back yard. It tooks us weeks. His neighbor (the Mrs.) was a retired teacher from my old elementary school. I had grown up with her daughter. She was estranged from her husband of 30 years. He had worked in a nearby factory that had shut down. Throughout that summer he would occassionaly show up at the house and they would argue, and then he would leave.
This particular day, he showed up and was talking to us for a bit about the deck, and the weather, and if we'd seen his wife. We had not. Later, he as moving furniture around on the patio. We realized much later he was actually barricading the back door from the outside, so she could not run out the back door. We didn't pay too much attention to him. It technically was still his house. He was there nearly everyday. None of our business. So, we just kept working.
A short while later, she had come home; and we heard him screaming from the front of the house, "Get out of the fucking car! Now!" Then heard his wife yelling back, "No, Just leave!" Get out of here, or I'm calling the police!" He screamed, "Get out now bitch!" So, we stopped what we were doing and ran around the house. We got to my friends driveway, and she was in her stopped car along the curb in front of their house. He was trying to open her car door. Before we could say anything, or think about intervening, he pulled a 9mm from the back of his wasitband, and fired three shots into the car. Their daughter had been in the car with her mother. She immediatley jumped out of the car, and ran across the street sceaming. Her father pivoted and fired two shots at her, striking her once in the back. She continued to run and went into a neighbors unlocked front door screaming. The car, still idiling, rolled up slightly onto the curb and rolled about 40 feet down the street before coming to rest against a mailbox.
After shooting his daughter, he swung the gun around and pointed it at us. We were both, young, athletic and each holding claw hammers in our hands! We were 20 feet away from him at best. He leveled the gun at us, then calmly said, "You guys don't know the Hell she's been putting me through." Then simply dropped the pistol on the ground. We both immediately ran past him and to the car. She was already dead, and slumped over on the seat. He just stood there.
There were tons of people outside. It was the summer. Lots of witnesses. I think someone called the police when they started to argue, because it seemed like mere seconds later a patrol car came around the corner of the street sideways. To this day, it was one of the best "drifts" I've ever seen. A huge cop in our town, they called "Big John", jumped out. John was about 6'8". He pulled his pistol, and screamed, "On the ground, fruit loop! NOW!". The dude dropped to the ground, John, walked around the car, and holding the gun with his right hand, picked the dude off the ground with his left, and slammed him across the hood and seconds later, he was in cuffs. Still with one hand John picked him up by his belt, and tossed him into the back of his cruiser.
His daughter survived, at least physically. The bullet struck her shoulder blade and fragments went into her thigh. But no permanent damage. He got 25 years to life. Looking back this guy was always known as being kind of creepy. He would leer at women and especially teenage girls. All of his daughters friends thought he was weird. At the local supermarket you'd occassionally see him trying to chat up a much younger female grocery clerk. He always gave off a perverted vibe.
Sometime in 2005 or 2006 his DNA was matched with an unsolved murder from back in 1974. Turns out, when his daughter and I were probaby 10 or 11, he had abducted , raped and killed a 15 year old girl who had hitched a ride one night after her boyfriends car had broken down, and he couldn't pick her up from work. He raped her, then beat her to death, and then dumped her body. He had bragged about it in prison, and someone ratted him out. The story matched the old cold case, and they later matched his DNA.
My friend and I testified at the first murder trial of his wife prior to the sentencing, as he'd confessed to the actual murder. He would have been eligible for parole I think in 2022. Not now.
But, yes. If someone was harming someone else, I'd assess the situation, call 911, and definitely get involved.