What to do

If a crime is going on and I ran into it I wouldn't be that person to walk away first I'll call the cops so back up would be on there way if their isn't a chance I stop it and the next thing i would do with out getting caught by the person inflicting the harm is put my phone somewhere out of site and record everything that happens. Finally I will try and stop him/her and keep the criminal there until the cops arrive as long as I can without getting myself but in case I do get hurt and the criminal gets away I will always have the video as backup