What if he had just attacked Guts and Zodds sword just completely shattered, this would show how far Guts has come, usually they would fight with Zodd in his human form for a while but now it's not even needed, Guts is far superior to Zodds human form now. So I see this as him holding the sword ready to throw it on the ground and transformHello Walter,
Sorry for the late answer, I had some medical issues.
As to your question, the idea was to have Zodd be like the guardian of the entrance to Pandemonium. For Zodd's broken sword, it was just for the composition, nothing too deep behind it I'm afraid.