Disgusting. The police are empowered to use necessary force to protect themselves and other citizens from immediate danger and to take suspects into custody. A man lying motionless on the ground is not a threat and is not resisting arrest.
They are not empowered to use force to dish out street justice. Everyone is innocent until proven guilty in a court of law.
I feel for the cops, and can imagine the emotions and adrenaline having seen a colleague being hit by a car, but that does not make them above the law.
2011 E92 M3, 2008 Evo X MR, 2008 S2000 CR