The Groomer referred to Shiki's power to see the lines as Mystic Eyes of Death Perception - maybe during the opening scene Shiki fell under a Dead Apostle's limited hypnotism of whoever killed his mother and that's why he lost control of his body and killed Arcuied? Or maybe Shiki became some manner of True Dead/vampire himself on that night and his bouts of anemia are really him subconsciously refeeding to maintain his form? That would explain why Yukimura apparently saw him roaming the business district past midnight.