Who is guilty?

Today as I was watching the news on TV I had to see one of the most brutal scene, a chain snatcher being beaten up. Read here http://www.ndtv.com/convergence/ndtv/story.aspx?id=NEWEN20070024158&ch=8/28/2007%2012:21:00%20PM
What truly disturbed me was the fact that when a cop appeared on the scene, he too joined the people to continue the brutality. Isn't that breaking the law? How can the very person responsible to maintain law and order, take the law in his own hands. The man made a mistake and should be punished according to the law, but the cop and the people who took the law in their hands should be charged as well for attempted murder. And that is justice.....