In the U.S., when a Muslim extremist commits a terrorist attack, it reflects poorly on all Muslims, but when a white Christian commits a terrorist attack, it doesn't get attached to all white Christians. Only the individual is to blame. Part of it is the majority gets to make the rules, but it also has to do with the idea of American exceptionalism. This has been true since Winthrop's notion that America is a "city on a hill" in the 1600s. Americans can't accept that the fabric of this culture can be linked with violent acts of white supremacy and Christian extremism. It's why Ronald Reagan was so popular with white Christian conservatives. His policies are not much different than John Boehner's, but he was able to feed into our self-perception as a moral nation.
If we blame a group for the act of an individual, we must do it with all acts. Dylann Roof is a symptom of a subculture that encourages the self-perceived victimization of the majority because minority groups are gradually asserting themselves as worthy of value just as Nidal Malik Hasan is a symptom of a subculture that feels marginalized and believes violence is the path to rectify the situation. Yet cultures do not commit violent acts; individuals do. Violence should create a moment for cultural self-reflection and dialogue with others, not hate-filled insularity.