Still not sure that failing your classes = mass murderer
Bread, people kill for a lot, lot less. In the top 10 motives for murder are jealousy, passion, revenge and obsession. I remember a case of a man who killed his girlfriend because she wouldn't give him her chopsticks! Or what about the bastard that stabbed a young girl in his anger at losing his internet connection?
Lots of angry young men kill for no reason, or simply to become infamous. Young men are indoctorinated into thinking a certain way, into hating a common enemy. I think it is likely that the older brother did this to the younger brother - and this was the result.