First, my actions in this scenario. On the assumption of intimate range, which I would've never allowed in the first place, I would engage immediately (though it would probably be with an edged weapon as it's quicker to employ than to try to draw and shoot). Even at distances that I would allow, if he continued to press the issue or advanced, I'd engage with a firearm. A threat of serious bodily harm with what is now a bioweapon has been made. A 0.3% infection rate may or may not be acceptable in a clinical situation, but out on the streets you have no venue for immediate action to prevent or hinder infection like a healthcare professional possesses in the clinic/hospital. What the person has done is threaten to murder me (though it's a slow way to go, I prefer not to go before my time). That constitutes lethal force, regardless of his threat turns out to be true (needle is HIV infected) or not (just some leftover smack in it).
First law of combat/war/anything, DO NOT UNDERESTIMATE YOUR OPPONENT. Feeling that junkies are easily overpowered is fine for hypotheticals, it doesn't always work in the real world. In the scenario, the person is assumed to be a heroine junky, that may or may not be the truth. Either way, under the influence of controlled substances, pain is felt differently and that person may be more difficult to subdue as originally thought. If that person happens to be a haggard looking PCP junky, you'll have a much larger battle on your hands than you thought and anything short of mortally wounding them will more than likely not stop them.
Next, what's to say that giving up your items will have the expected outcome. Obviously if the guy has a threat that works on you for a few items, why not press for more. If he is willing to make such a threat on your person, what is to say that he is telling the truth that he will leave when he gets what he originally demanded?
I don't carry a taser, so my only options are hand to hand (not willing to engage due to various reasons, but most of all due to the assailant being armed with a potentially deadly weapon and I'm not into suicide at this time) and escalation to lethal force (edged weapon--not happening as I wouldn't allow him into my comfort zone-- or firearm). He's getting shot.