About 50 cops are shot in the line of duty in the US every year. There are no really good statistics on how many civilians cops kill, but it seems to be a much larger number, about 1000. A spate of recent killings of unarmed black men has drawn public attention, but they aren't the only victims. Cops kill a lot of unarmed civilians of all races, but it seems likely that the victims are disproportionately young and black.
My guess is that hardly any cop goes out on the job with the ambition of killing anybody, much less an unarmed civilian. Such killings are tragic and usually avoidable. Victims of police violence want the perpetrators punished, but they rarely are. Grand juries, and especially prosecutors (who have to work with the police), are very reluctant to indict, especially when there is reason to believe the victim was resisting.
The first instinct of a policeman when faced with danger is probably to reach for his gun, even though he has less murderous options on his belt. Why? Mainly because he doesn't want to be the guy who brought a billy club to a gun fight. We live in a society where any idiot can own a gun, and many can carry concealed quite legally. As long as the NRA runs the country, cops are probably going to be quick on the trigger.
The prosecutor handling the grand jury that failed to indict Eric Garner's killer apparently did not give the grand jury opportunity to consider some lesser charges which may have been more appropriate. And, of course, it's Staten Island, where:
It is a place where its lone congressional representative, Michael Grimm, faces a 20-count indictment, threatened to throw a television reporter off a balcony, and still won re-election by ever larger numbers.