The following Letter to United explains this review:
This is to complement your flight crew, and also to report the unpleasant incident that happened: The male caucasian passenger (traveling with a 10 yr old girl) next to me in seat 23B said the arm rest was normally for the person in the middle seat to use. My response was: "I've never heard that, I believe you're wrong." He disagreed (I don't remember his exact words at that moment) but he became more agitated. I responded saying: "I've been flying many years and am sure thats not true". About 10 seconds later, he hit me with a sharp elbow in my right arm. I asked: "what's wrong with you? That's not ok." . His angrily responded while making a fist close to my face and said: "listen, if you don't stop, I will f--king pop you in the head!" That's when it was clear he was an angry unreasonable person and I became concerned for my safety and pressed the call button to report what happened. Fortunately I thought to surreptitiously take his photograph. The flight crew got involved and moved my seat.