When Arn first learns to set the Silver Sword on fire, he is taught the key to igniting it is a pure, passionate emotion. Arn mistakes this to mean anger and outrage were the only emotions great enough to access in battle. At this stage of the story and Arn’s abilities, tapping into such self destructive emotions to activate this power actually produced burning pain in his hand and arm the longer he uses it. Anger harms the wielder as much as its target. It’s not until later that he finds a solution in the place he did not know to look.