Zed was never really fighting for demonkind.
Zed's a Harmless Villain, but when he actually uses his full strength, he's stronger than any of the Quarter Knights. Is it just because he's attached to his sword and his Bishōnen form, both of which weaken him? No, it's because he's not actually interested in conquering Filgaia at all. Humans won't accept him because of the whole demon thing, so he sticks with the demon army, but he's intentionally sandbagging against fighters who are strong enough to tackle the Quarter Knights because he'd rather see humanity survive and win.
