So the scream makes a difference?-
what exactly is the difference when both are killing innocent people?-
tell me?- -
maybe the bullets a white racist fire hurt less?-
No both are killers are for sure, but the motive behind the killing gets you the label. Anders Breivik was called a white supremacist, cause that was the motive behind his killing. Simlarly the Islamist militants are driven by religious motives (Atleast thats what they think anyways), so they get the label of Islamic Militants.
Heck the real name of Boko Haram, the militant group behind this particular carnage, is Jamā'at ahl as-sunnah li-d-da'wa wa-l-jihād .