User blog comment:Xilinoc/Ch. 643 - BABY,HOLD YOUR HAND 6 (Waiting for Love)/@comment-24503264-20150917045558/@comment-24153155-20150921165524

I guess he is dead, but not dead... euhm, yeah... Technically, Kurotsuchi stabbed him into his heart already, but because of the superhuman drug it may take a 100 years for Szayel to catch up with that (manga chapters 305-306). Even if Szayel's ability to resurrect would work, I suppose it would take another 100 years or so to activate, meaning that his body has long since decayed.

Everything of course is possible in manga, but personally, I think it would be very strange for the story development if yet another slain guy goes zombie or even gets reborn in some funky way. However, I do remember Kurotsuchi and Nemu found Szayel's lab storage room...