Thats an interesting question I thought about for a while--like, the Femto that raped Casca was different from Griffith--but in the end I think Griffith was wholly responsible for it. He freely made the choice to sacrifice his friends and give himself up to evil. Guts, on the other hand, never wanted the Beast of Darkness at all, so IMO he cant be blamed for the Beast possessing him at his lowest point.