Genocide is only a crime if there are cameras.

As the Jewish Encyclopedia put it, "David waged a sacred war of extermination against the Amalekites," who subsequently disappeared from history. Long after, in the time of Hezekiah, five hundred Simeonites annihilated the last remnant "of the Amalekites that had escaped" on Mount Seir, and settled in their place (1 Chr. 4:42-43).

The Biblical relationship between the Hebrew and Amalekite tribes was that the Hebrew tribes hated the Amalekites, primarily due to Amalek's attacking Israel on their way out of Egypt.
-http://en.wikipedia.org/wiki/Amalekites#Genocide_of_the_Amalekites