I like to think that the Skull Knight is one of the sacrifice that manage to got his hand on Behelit and able to survive, idk if that's possible for a sacrifice to use it. But he might be a struggler like Guts, and thus he also try to kill the God Hand just like him. But he's a former Emperor, so i assume someone might use the power of Behelit to kill him, like the queen or his comrades and sacrificed the citizens for revenge. BUT THAT'S JUST A THEORY, A SHIT ASS THEORY.