Time has proven a surprisingly high number of innocent people have been convicted and if it's a capital crime, the defendant pays for such a mistake with his life.
Making sure of it's verdict? They went well beyond that. Beside the fact that if found guilty there can always be an appeal, but once acquitted double jeopardy goes into effect, I'm pretty sure they have yet to find anyone that was later proved innocent that received the death penalty.
This movie is a pure societal statement from beginning to end. It drives me crazy.