Based on what I know I'd have drawn and escalated my threat level based on their actions. I was threatened with a deadly weapon therefore I display threat in order to remove threat of death or severe bodily injury. If they further advance their actions I squeeze the trigger until the threat is not there anymore and call the local authorities.If they back down I hold them at the scene with the firearm at the low ready and phone authorities but never take your eyes off them. File a report with the 911 operator and inform them that you are armed and have the attacker at gunpoint and give brief statement stating that after a traffic accident they pulled a knife on me and hopefully the officer that arrives on scene isn't a shoot first and ask questions later type of guy.
And when he arrives state just what you told the operator and ask for a lawyer.