Police are trained to recognise signs of imminent violence including (from national police college):
facial expressions
increased or prolonged restlessness, body tension, pacing
general over-arousal of body systems (increased breathing and heart rate, muscle twitching, dilating pupils)
increased volume of speech, erratic movements
prolonged eye contact
discontent, refusal to communicate, withdrawal, fear, irritation
unclear thought processes or poor concentration
delusions or hallucinations with violent or aggressive content
verbal threats or gestures
reporting anger or violent feelings
blocking escape routes.
At least 6 of these and possibly more were present. And they were fairly certain these people had already been involved in a very violent incident. The situation had escalated far beyond polite request not to point at him.