Question.
Why would we hold a trial? It just kills us off faster, after all. And runs the risk of getting everyone killed. For that matter, if, like Monobadger said, the wrong person is accused, and everyone dies... Well, it sounds like the murderer does too.
The murderer doesn't die if everyone gets it wrong, and the murderer gets to go home free; Monobadger and the rules themselves state this. There's your incentive to murder.
As to trial? If you don't, the murderer gets to kill more of you anyway. The trial forces you all to stick in one place, where nobody gets murdered without being in plain sight of everyone. At least with the trial, you have a chance of stopping the murderer without being a potential murderer yourself.
And as everyone says, wouldn't be any fun for Monobadger.