Obviously it depends on your states laws - you'd be stopping him from aggravated robbery, justified in Texas with the use of deadly force. If you live in a liberal county *sigh*, I don't know how well this would go over with a jury. I think it's an easy call, he's commiting armed robbery on you - but with a crappy weapon. You pull and let him know what's going to happen if he moves while you wait 30 minutes for the cops to arrive.
He makes a lunge with a sharp, pointed, metal object at you - it's an easy call.