I am assuming you mean who decides which truth is closer to the truth?
But truth for whom? A priest may believe it wrong not just to kill, but even to speak a harsh word; a soldier may believe it justified to kill in defence of home and kinfolk etc, but not to shoot a man in the back. A thief might believe it justified to steal to feed his family, but not to take from friends, etc.
Every individual has a personal code, and it is only when he steps outside his own code that he feels guilt. It is the person who doesn't have any personal code, that is truly frightening, imo.
We have societal laws for the good of society, but they are only that. They are not individual moral judgements.
Those come from somewhere else. Who decides what is spiritually right for me? I have a choice to listen to all sorts of views and religious tenets -- often they differ in the detail.
I accept I abide by the laws of the land I live in -- but I totally reject that a spiritual messenger dictates the terms of my existence, and can exclude me from the common rights or dispensation because I differ.
Yes, it's been tried in the past. But it was wrong then too. And the past did not have the past to learn from.
So ...