Gissur was standing his ground against Sturla, the leverage they thought they would have with capturing his family was nonexistent, Gissur was an extreme person and knew he would reunite with them in Valhalla.
Sturla was here, the elusive Sturla, and this was his chance to strike a vital blow to Sighvatur, or would it be more accurate to say the Sturlungar clan?
Sturla was surprised that their plan did not go as expected but his father had considered this and still accomplished their end goal; did they just want Gissur's wife?
That seemed a bit short-sighted but it was a possibility nonetheless.