The man thought he managed to outsmart Zaen, but the minute he abruptly started running, Zaen was sure his cover had been blown away. That's when Zaen hurried put on the mask of secrecy and chased after the man. Now that he had gotten so close to unveiling the truth about ghouls and everything else, he couldn't let someone insignificant screw him over. It didn't matter whether the man was innocent or not.
He could have used his daggers or guns to get rid of the man in a flash, but the white decor of the room made Zaen rethink his choices to eliminate the scientist. He was afraid that if even a drop of blood was spotted in the place, the Special force would know something was going on there. Thus he was only left with the option to do a silent kill.
