Who was the first person Kazami killed?
The two people that Kazami killed were apparently named KANEKI Kyouko and MATSUI Aki. Here's a picture of them together:



Right after the above shot, Kyouko (left) falls to the floor, apparently stabbed in the back by Kazami. He then chased after Aki (right), which is why she was running when she bumped into Sakakibara. Here, we see Sakakibara coming across Kyouko's body:



Shortly thereafter, Aki runs into Sakakibara's arms.



Then, Kazami stabs her in the neck, raising his kill count to 2.