Wild scenario....but anything is possible. If he started to run at me like a mad man I would probably just dodge him and let him run around until he bleeds out. Then call EMS. I couldn't justify a shot given that he dropped his weapon but if he didn't then it's game over for him.
Duck, Duck, Goose!
I would say for legal reasons never shoot a guy that is coming towards you like that. HIV is a difficult virus to transmit, your chances of contracting is low. Also, you can take antiviral meds that prevent developing an HIV infection after being exposed to the virus, immediately contact your doctor and a pharmacy. The take away message here is that just like people overestimate the lethality of bullets based on movie renditions, people overestimate the infectiousness of many (not all) communicable diseases.
If you shoot a man that you *suspect* may have HIV/AIDS and is running towards you, that does not justify a deadly threat, you do not have the legal right to shoot him. If you do, you will probably go to prison for some number of years with serious repercussions on your family, future job prospects being an ex-con, and your risk of contracting serious diseases in prison including HIV, TB, hepatitis, MRSA, pneumonia, and many others. All for what? Some crazy motherf that has AIDS? That **** is stupid.