This is the issue with case lawAgain, what is 'bloody obvious'? You do have to have a boundary somewhere to determine what level of evidence constitutes whether something is obvious or not. We are back to the idea of deciding what that arbitrary boundary is, and assuming that the police, juries, judges, prosecutors, and expert witnesses don't make mistakes and/or skew the process. Judges have been know to make mistakes on many occasions...
Once one innocent person is wrongly executed, the whole process becomes unjustifiable. That is too high a price to pay for vengeance.
How on earth are those boundaries set?