Please show where in the Constitution it notes where citizens can be accused to crimes without trial and shot, because the majority of the BoR pretty much says the opposite.
If a person is actively attacking a police officer (or anyone?) with a deadly weapon, current law does not require even an accusation, let alone a trial, before police/civilians are allowed to shoot them in self-defense and/or to protect the lives of others.
Preventing such occurrences from happening by disallowing police/civilians from shooting people who attack them is not a good idea - it would only lead to police dying more, not to mention civilians who carry also dying...That, or going to jail for defending their own life, which is not something that should be illegal IMO.
Rather, the optimal way to prevent such is to stop the person from reaching the point where they are attacking police/civilians.
To do this we need to improve the access to and availability of health care for all issues, but especially mental issues.
Additionally we need to cut WAY the **** back on imprisoning people for petty, stupid, things.
Prisons have a tendency to make some people MORE prone to crime, rather than the reverse, simply because of the environment they are placed in.
Placing a person in prison for something that did no real harm to anyone except possibly themselves is not acceptable.
Edit: In short, the solution to the problem you see - police/civilians killing people who are committing crimes - is NOT to prevent the killing, but rather to prevent the crimes that IMO often completely justify the killing.