The comment and threat of Boga touched the reverse scale of Zael. He had always been a calm person. Someone who does not engage in fighting that easily. Throughout his life, he had heard countless criticisms and threats, but he never really paid attention to all of those. He only takes action when he deems the threat to be valid. However, he valued his friends greatly. Even empty threats towards them trigger his wrath.
As a matter of fact, the ancestor of the empire that he destroyed back in his universe merely gave an empty threat to one of his friends. Alas, this triggered the wrath of Z and the other members of his group so much that it prompted them to wipe the empire out. Even though he can be dubbed as powerless at the moment compared to his previous might and stature, this did not discourage him from removing the life of Boga.