What does Matt Stern regret about the day that James died?

Matt Stern was the medic who tended to James after he was injured. Matt ran to James's Humvee, opened the door, felt for a pulse, grabbed James, and pulled him out and saw the extent of his injuries. Matt kept looking at the left leg that hung by just a piece of skin. Matt knew it needed to be cut off, but he could not bear to do it. Although he worked hard to save James, Matt still regrets not cutting off that leg.