Why can law enforcement kill someone without a trial?

It is my belief that a death caused by a law enforcement officer goes through a comprehensive investigation including court hearings, and the law enforcement officer who committed the death of a person is suspended with or without pay as the investigation unfolds. Law enforcement officers are there to protect us; their jobs are dangerous; and many have been killed head-on by violent perpetrators as they go into dangerous situations trying to protect the public. Remember that they are chasing people who are armed and dangerous most of the time; and they are targets also. Are there some dangerous and unethical law enforcement officers who potentially take advantage of their position, or make horrible decisions that resulted in fatalities that shouldn't have happened? Yes, and through the investigation process, the law will hopefully weed them out by trying them for the crimes they commit. Justice is suppose to be blind and applied fairly to all. Is justice always just? No, because we are human and make mistakes. The mistakes can cause injustices that are tragic, but crime is tragic in all senses. I fully appreciate the law enforcers who protect me.