"Isn't racist killing a federal crime?"

Crimes of most types are a State responsibility to prosecute. The races of the people involved are often coincidental to the fact of the crime itself.

http://en.wikipedia.org/wiki/Hate_crime_laws_in_the_United_States

That link deals with hate crimes.

http://www.answers.com/Q/What_is_the_difference_between_a_federal_and_a_state_crime

An easier read than many sites on the net.