God forgets again?

Deuteronomy 19:3 says, "Thou shalt prepare thee a way, and divide the coasts of thy land, which the LORD thy God giveth thee to inherit, into three parts, that every slayer may flee thither."  Didn't god write a commandment that said "Thou shall not kill?"  Did he forget that he made a blanket statement?  Now he just shifts killers around to different cities. Guess he never heard of a serial killer?