Im probably wrong, so dont quote me - But there was always a focus on Griffith's 'hawk eyes' when he wanted to to kill someone, such as Adonis, or when he wants something. His pupils become like a cats, or 'hawk's', very straight and narrow. Maybe the lightning bolt shape of the Beast Of Darkness symbolizes the unstable, overwhelming animal-like nature that the Beast Of Darkness has, in comparison to Griffith who is very cautious, has clear goals, and is thorough and controlled in his actions. Just some quick thoughts, but either way the Beast Of Darkness's eyes are cool as hell.