Screw that. In reality, there is no such thing as certainty in religion and morality, because there is no underlying reality to be uncovered. It is not that we do not know our position is "right", for what would it mean for it to be wrong, unless you can establish some kind of objective existence of morality. Likewise, you no more know that it is "better" to not do something to other people because of one's belief in a moral system because you have no better authority to appeal to than they do.
No certainty - only conviction.
Value is central to living. Second guessing is just creeping nihilism.
Now, you might say, from the subjective stance of a Christian there is in fact some kind of reality to be uncovered for right/wrong valuations because there is this notion of God. But then, since we can apply likewise a possibility of the judgement of God for any moral proposition, which would then be equally uncertain, stating that uncertainty implies inaction is a sort of false dichotomy. Perhaps God hates people who don't do things.
Of course, you have your 'evidence' that certainty leads to atrocity, but this fails because You have failed to demonstrate that the good that can be caused by individuals who believe whole-heartedly in the goodness of their work (of any kind, mind you, we need only that they have absolute certainty in the value that drives them, from social equality to God-knows-what) is in any way outweighed by the occasional organized atrocity. Maybe you can say, atrocity=>certainty, but then what of the converse?
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* here for the nachos.