The media portrayal of police officers as negative or positive depends on which newspaper, news magazine, radio, or television news program reports incidents. For example, if one listens to Al Sharpton's appearances on MSNBC or his radio program regarding the shooting of Michael Brown in Ferguson, Missouri, then the police are virtual vigilantes.
Speaking at his weekly National Action Network rally in Harlem, Sharpton panned Wilson's claim to be in fear of his life as the "same excuse" as others who fatally shot African-American teens.
In a positive report toward Officer Wilson, two forensic experts reported to Fox News that there were scientific indications, such as powder burns on his hand, that Brown had wrestled Wison for the gun when he was in the police car. Here is another report from the St. Louis Post-Dispatch that confirms the one reported on Fox News:
The official county autopsy...showed Brown was shot in the hand at close range based on the finding of “foreign matter ‘consistent with products that are discharged from the barrel of a firearm,’” in a wound on Brown’s hand, the Post-Dispatch reports. "[This] guy is reaching for the gun, if he has gunpowder particulate material in the wound,” Dr. Judy Melinek, a forensic pathologist in San Francisco, told the Post-Dispatch.
Since the mainstream media thrives on sensationalism, negative portrayals of anyone usually gain more attention, especially if there is a racial issue; therefore, the overall depiction of police is probably more negative than it is positive. Certainly, in the turbulent 1960's there was coverage of police brutality when Mayor Richard J. Daley of Chicago ordered his force "to shoot to maim or kill arsonists" in the riots of that city. Then, there is the infamous coverage of Bull Connors' orders to hose Civil Rights marchers in the South.
On the other hand, there are sympathetic and congratulatory depictions of police officers who are heroic and who have died in the course of duty. Extremely positive coverage was given to all the courageous and self-sacrificing firemen and police officers on the tragic days of 9/11 and the Boston Marathon bombing, for instance.
Depending on the news story, they can be portrayed as both positive or negative. In my experience, I always hear about police in a negative way. I always see stories about cops hurting innocent people or making false judgement, but it is rare I hear a story about a cop doing something good, even though a lot of them do. The media portrays them in a way that will get the most attention. Not every cop is the same and they are trying to do their jobs, but the news only wants a story that people will watch/read. So, for the media, cops are seen as negative, whether that is true or not.