Although Rad was smiling, that was just due to him enjoying the fight. He had every intention to kill as many of the humans here as possible. As a blue core beast, it was rare for him to have the chance to fight and kill another beast. Like the humans the lower ranked beasts were able to kill each other, but the stronger beasts do their best to avoid killing each other. Yes, they fight, but if even one of them were to die then that might affect the delicate power structure between the beasts and humans.