One of the Ways Moral Relativism Defeats Itself

Bruce Bawer in Death by Entitlement quotes an NYT commenter:

One reader comment, a “Times Pick,” read, in part, as follows: “A great story and an admirable couple. But those who condemn their killers as evil probably fail to recognize that ISIS fighters see themselves as being on the side of good. For them, these young Americans were an embodiment of the Great Satan....Instead of bandying around moral absolutes, perhaps we should recognize that good and evil are relative categories, dependent on your culture and your values.”

Suppose that good and evil are not absolute  but are culturally relative. Why should we recognize this?  And why shouldn't we impose our Western values on benighted jihadis?

For the argument in full dress, see A Relativist Cannot Rationally Object to the Imposition of One's Values on Others.