In criminal trials, there are, I take it, two things which must be determined: matters of fact and matters of law. That is, answers to two questions must be provided in order to ensure a fair verdict and sentence for the criminal: 1. What actually happened? 2. What point of the law applies?
The scope of the first question may be narrowed in a criminal case: Here is the crime of which the accused is accused. Did he actually commit that crime?
The second question: Granted that he did that of which he is accused, how does the law apply to his particular case?
Due process ensures that the criminal is treated fairly, and that these two questions are answered fairly and adequately.
For matters of fact, there are juries.
For matters of law, there are judges.
For the execution of sentences, there are executioners.
And due process is perfectly superfluous in at least some cases, namely:
When the matter of fact is evident, because the police officers are witnessing it happen, or because there is an abundance of witnesses or physical evidence, and the mental state of the criminal is not in doubt.
Consider the following case, for example: Suspects have just robbed a bank. Police arrive before the suspects can escape. Suspects fire shots at the police.
There's no question of fact. The police are certain that the suspects are shooting at them. There's no question of matters of law: 1. Bank robbery, 2. attempted murder of policemen.
There's no need for separate judges, juries and executioners. Once the police apprehend the suspect, they should be able to be judge, jury and executioner right there. The suspect shouldn't even make it to the police station alive.
In such cases where the facts are manifestly evident to the police officer, the only trial that a suspect should receive, if he should receive one at all, is in an appelate court.