Usagi has a distorted mental state. Upon first glance, he seemed to be quite childish, careless and impulsive as well. Despite all, he is quick-witted and strategic. As seen when he purposely kill Snake before the tournament starts, this allowed him to gain an advantage to face others with his necromancy. He is always thinking ahead, preparing for his next battle. So, when he kills he does so carefully so that he doesn't cause too much damage his new "friend." 