Freetrader, I pulled out the old Rodney video to show a famous if somewhat old example of what happens when a lack of police restraint goes to far, judging by youtube results there are plenty of incidents of police violence, many with fatal results. Whilst cops do indeed put their lives on the line, it doesn't give them a right to practice a little whoopass.
I thought the Constitution also protected against actions without probable cause (4th amendment), without due process (5th amendment) and prohibits cruel and unusual punishment (8th amendement). I also thought many law enforcement officers also had a duty to protect and uphold the constitution. Beating someone to death in a DUI checkpoint seems to fail on all three.