This might be one of the simplest Bergmanian films but simplicity is not necessarily bad, in fact everything is carefully assigned with meaning, and it is better to analyze the film on a symbolic and allegorical level. btw have to give credit to the cinematography, it is amazing.
This is a battle between reasons & emotions, self conscience & personal prejudice, life & death. Their "reasonable doubt" manifests basic respects for life and the protection against abuse of individual right & freedom under the judiciary system in a democracy.