It seems to me a fatal shooting should always be subject to a court hearing. I think that the killing of a person should be subject to an open and impartial review. Decisions behind closed doors e.g. the Sherif deciding to take no action, or the prosecutor deciding not to prosecute have the danger of being seen as corrupt.
I think the shooter should have been promptly arrested and interviewed to get an immediate record of what happened (with counsel if desired). The shooter needs to be breathalysed and checked for drugs (the shooting victim will be). The gun needs to be checked (just in case there is a claim it fired itself), the gun license needs to be confirmed, any training by the shooter recorded. The shooter does not necessarily need to be detained if he is not thought to be a danger to others.
If the shooter was intoxicated and judgement impaired this would be an important issue. If the shooter was diabetic and their glucose was low and judgement impaired this would be an important issue.
If knowing that you will have to go through a court process if you kill someone acts as a deterrent for shooting then I do not think this is a bad thing.
Yes I think this should include fatal shooting by cops. In simple cases the court hearing might be fairly brief, the bad guy had his gun out, he was shooting into the crowd I issued a verbal warning and then shot him. Case dismissed.
The point is a killing should be openly and publicly justified. (grand juries as I understand are not open and one sided so would not count). In England any death such as this would alwaysgo before a coroner's court* for an inquiry (not a criminal court), evidence is given, witnesses can be questioned, a jury gives a verdict. (There are many verdicts possible including narrative verdicts, but common ones are suicide, misadventure, justifiable homicide, murder). Even a verdict of murder by a coroners court does not result in automatic criminal proceedings, the burden of proof differs, but would be likely to be followed by prosecution authorities, even if a previous decision not to bring a criminal case had been made.
* If there is a criminal court case this can also function as the coroner's court inquiry but not vice versa.