There is no context which can justify the police punching someone in the face and then walking away.

Because they are supporting police brutality. Their comment and replies are dripping with the implication that they want context so they can judge the officer when the situation doesn't need explanation at all. It's a tried and true tactic to make bootlicking more acceptable.

Edit: See also: "We need context before we can judge the Israeli Wehrmacht for literally shooting children in the head with a sniper rifle."