Kk. Simplified in such a way my 11 year olds could understand. The rules only work if they're equally applied to all suspects and criminals. Once a single review of evidence becomes enough to kill a person, it becomes enough to kill the appeals process altogether. What with how dead people don't get appeals.

Due process- a course of legal proceedings according to rules and principles that have been established in a system of jurisprudence for the enforcement and protection of private rights.

So, to be clear, all suspects and convicts are given the same rights. The court system works off of existent precedent quite a bit. So those rights aren't just established by some written-as-a-whole document (though that exists per state and federal level) but also by what courts have decided in the past.

There's no way to do that without shredding due process, unfortunately.