Yes the police have a job to do. It can be a hard job and it can damn sure be a dangerous job at times. But just because you wear a uniform with a badge and carry a gun doesn't mean you have carte blanche to decide who lives and who dies.
Last August in Sherman, Texas (about an hour north of Dallas), Mary Surratt was stopped for not signaling a lane change. She was arrested and placed in the back of a patrol car without being searched. While in the back of the car, Ms. Surratt tried to ingest cocaine she had on her person.
An unnamed police officer then began hitting Ms. Surratt with his fist and a flashlight to try to get her to spit out the cocaine. When that didn't work he pressed his flashlight against Ms. Surratt's neck until she passed out. The officers then took her out of the car and tased her while she lay on the ground unconscious.
Ms. Surratt lie on the ground for more than 20 minutes before she was taken to a local hospital where she was declared to be in a vegetative state. She died less than a month later.
By all accounts Ms. Surratt broke the law. But even had she been convicted of possessing cocaine, at most she would have ended up in state jail - more than likely she would have landed on probation (or even deferred adjudication). She certainly wouldn't have been sentenced to death.
The officers involved in the stop and detention (and death) of Ms. Surratt might have been pissed off that she was getting rid of evidence in the back of their patrol car - but that didn't give them the right to cut off her air supply and kill her.
Why did it happen? I don't know for certain but I would guess that it has something to do with the current military vibe in police departments across the country in which the police go out into the street with an "Us versus Them" attitude. Once the police take the view that it's them against the world, the notion that the police are there to protect and serve goes out the window. When that happens someone is getting the short end of the stick when they don't jump high enough for an officer.
Ms. Surratt was that person last August. Whether anything happens to the officers involved will tell us whether the City of Sherman is more interested in protecting and serving or in waging war on "them."