Be it the brother, father, the grandfather, teacher, or whatever - a lot of shounen anime (and other genres) have the protagonist lose their mentor. And usually the mentor is an extremely feared, respected figure - probably almost all of them are the top ranking powerful beings in their anime.
Why do they have to kill off all the powerful mentors anyways? Most of them are extremely likable - and wouldn't the story have continued better? IMO, the protagonist would've developed better with the mentor ALIVE - someone needs to teach the kiddy kid anyways.