What do you do when you are assaulted by a bouncer?

I went to a bar with my friends. We were just bar hopping and got around to our last bar. We were in the bar socializing and having fun with other patrons. Out of nowhere the bouncer approached us and said we need to buy a drink or leave. I told him I don't drink and he walked off. Awhile later he approached me and grabbed me and kicked me out. I asked him if I could collect my friends and we could leave together. He didn't allow me to do that. So I walked around to wait for my friends to come out. I could see my friends from the window, they had drinks in their hands and the bouncer approached. He looked very angry so I watched him. He looked like he was rushing them. Moments later he took the drink out of her hand and threw the drink in her face and the cup in her face as well. I ran back inside the bar because I was afraid he would hurt her. When I got in her face was bleeding. I couldn't say anything but ask him,"why he would do that to her?". The look in his face you could tell he knew he was wrong. We called the police but only received an incident form. I never experienced anything like this but I wondered why he would react like this. If he would have allowed me to collect my friends she would have never been hurt. Please tell me what we should do. Thank you.