By early Tuesday afternoon, no one but Nia Wilson’s killer could know for certain why Wilson and her sister were singled out of the crowd, stabbed in the neck, and left for dead on a BART station platform Sunday evening. But for many who mourned Wilson’s death, the evidence had already stacked up. The randomness of the attack, the rumored meeting of a white supremacy group in Oakland, and the races of a black victim and alleged white perpetrator all carried the trappings of a hate crime.