His brother had too die, so that now he can slaughter a bunch of people, who don't stand a chance against him? I have an idea, where this might have been pulled out from. Well, It does smell like fart. But I've read worse.
Welp, even if it was quite the evil one-sided slaughter and taking 200 lives for one doesn't really add up, it is what it is. An rebellion for no other reason, than to attain power brought up and lead by disgusting pigs. And those mages and also fighters (I guess?) decided to side with them. That's what they get for treachery. Still no reason for the good brother to die. Even worse. If he would have just killed his father and bastard half-brother with two fast-cast level 5 spells, ... Than those 200 bad apples might have survived. Maybe it is better to wipe them.
Also
@Sakaiy: Seconded. Shit-paced manga.
The manga is still not horrible at this point considering all chapters, though.