@STING380: it was the Skull Knight line to Guts about how the world is a river, the Causality is the moon and the human struggling in that world is merely a shadow of the moon. No matter how the moon reflection changes its shape, the moon stays the same. The shadow will always be a shadow, it cant escape the water surface, just like Causality is inevitable no matter how humans struggle. Guts, on the other hand, could be a fish that can leap out of the water, and may have the ability to change the Causality itself. There are lots of interpretation for that dialogue, but that’s my take with it.
The decision of Guts saving Casca in this scene also takes us back to Skull Knight’s talk with Gus about him having to choose between protecting Casca and continuing with his revenge. His action interrupts the Causality in progress. This action may still falls within the boundary of the Causality, as a fish can only breaks the water surface for so long before it falls back. However, it still causes a disturbance to the flow of fate, and more importantly it serves a big role in Guts’ character development. He finally found his answer, he’s the fish that breaks the water.