Cullen Murphy, in an essay in the Times, says that doubt and uncertainty are natural and inevitable parts of the honestly viewed human condition:
That’s the way it is with moral certainty. It sweeps objections aside and makes anything permissible if pursued with an appeal to a higher justification. That higher justification does not need to be God, though God remains serviceable. The higher justification can also be the forces of history. It can be rationalism and science. It can be some assertion of the common good. It can be national security.
Those who are completely sure of themselves usually exclude the possibility that they can err, or that others can be right, or live virtuous, decent lives involving another belief system. They find it easy to deny a common humanity, and to accept collateral damage in pursuit of what they see as unambiguously good. History, in general, hasn't been kinder to them than they have been to their fellow human beings.
One of the moral absolutists' defenses is that without such absolutes--often, but not always, arising from religious orthodoxy--we're left in a land of situational ethics, of moral ambiguity where anything goes. I disagree. One is still compelled to act as decently as one can. The uncertainty with which the honest person confronts such actions requires, in fact, a mindfulness, a sense of personal responsibility, which orthodox true believers run from and reject. Erich Fromm's 'Ezcape from Freedom', which I cite all the time, is on point here: one response to doubt is fear; one response to contingency is the artificial imposition of order on a chaotic, unknowable universe. Neither is the best of which human beings are capable.
Those who call for increased personal responsibility only from others, not starting with themselves, are a walking oxymoron. Not everybody has the courage to be uncertain.