Eh... I don't know... If Bayonetta couldn't kill Dante, why would Sephiroth be able to kill Vergil? Bayonetta killed angels and gods. Sephiroth... Killed Aerith. With a surprise back-stab. But other than that, he doesn't really accomplish much... And can obviously be destroyed by three relatively normal guys. Also, that full-to-1-hp attack?! It can be interrupted )well, in Kingdom Hearts, anyway).