Filming or photographing people without their consent, and I don't mean random people walking by on the Mall or attending a concert, is an invasion of their privacy. You're not Batwoman. You have no idea what the complete situation with these 2 people is, or what they are capable of, and you have no legal authority to interfere in their lives. If there is violence or a threat of violence, by all means involve the police since you are not trained nor equipped to deal with violence in any way. A smartphone camera is not magic.