No, a unanimous verdict is required.
There would definitely be an inquiry, especially if there were no other surviving witnesses, though in this day and age, with the prevalence of CCTV cameras, it's hard to imagine there being no witnesses left at all.I want to move this away from the Chicago killings, because we're debating general principles now. I agree that someone who killed an armed robber in commision of the crime would appear to be a hero - but even he should face some form of inquisition or trial to ensure that it really was an armed robbery and that he wasn't either an accomplice masquerading as an innocent party, or the person who initiated a fire-fight in which everyone else got killed.