I'm not a fan of FAMILY GUY either (or, really, any adult cartoon for that matter), but I AM aware of some of the behind-the-scenes drama that seem to plague it, and I'm almost guessing that killing off Brian is probably some kind of passive aggressive stunt Seth is pulling to get the message across that FG needs to be put to rest already.
It's not too far-fetched; I've heard of other stories where creators/producers want to put an capper on their shows, but networks wouldn't let them, so they would start sabotaging their own shows to get them canceled. Heck, in M*A*S*H's case, CBS kept renewing the show year after year because they didn't want to lose it, despite the cast and crew growing tired, running out of story ideas, and wanting the war to end, and so basically, just about the entire cast and crew went to CBS and said enough already, let it end, so finally, their eleventh season turned out to be their last, with the humongous series finale and everything.