As a law enforcement student I always hear of how police do countless amount of police brutality but people seem to forget that officers have their view of the situation. for example dealing with a man who has his hands always in his pocket and countlessly staring at your abdominal area, people may not really see this as a threat but to an officer this is a possible threat. Many officers fear that once the person has their hands in their pockets they may pull out either a knife, a gun, or even a crowbar. Having being in a law enforcement class I've watched many videos and it's very understanding to see why considering a lot of officers on duty have been face to face with this problem.
The link shown will show you why officers take caution: https://www.youtube.com/watch?v=Ade7dO8dmd4
Though there are cases where an officer may abuse this excuse, the threat is still very real and is out there to look at. In the line of duty officers may need to kill not because they want to kill but because they live by a code to go home at the end of their shift and if not for that code then may officers would not be alive to this day. For instance the shooting of Micheal Brown, Officer Darren Wilson had engaged Brown but during interface with Brown the officer had engaged in a struggle where Brown would attempt to take the officer's side arm but was killed before doing so. Officer Darren stood trial for the incident but was deemed not guilty due to it being in self defense. Even with people saying that the case decision was crap, until someone personally steps into a situation where they have to fight for their life then they don't understand the true pressure.
Another example is the encounter of Officer Johnna Watson. From policeone.com, Officer Watson had been called to a simple trespassing crime but had escalated to out fight. She had simply come up to him and ask him to leave the premises of a residence's home in Oakland California. The Homeless man had then gotten angry and struck several times in the head with a heavy chain and horribly injuring her but before she could black out from shock and be rushed to the hospital she shot the man and saved herself from dying that day. So it all comes down to the decision and a will, it's in our hands to survive another day or die from not reacting,