She was Naoya's target. From what I understand, he made an attempt on her life before and failed. This time he didn't want her to slip away.

A man of Naoya's level of madness cares little for life or anything that represents the force of good. Kohaku is an angel, a living symbol of holiness and light. To him, extinguishing that light brought him a powerful sense of self-satisfaction and an elevation above that of a mere mortal.

[...Scared yet, Ginko?]