Well first off it is not illegal (US) to take a picture of anyone on the street regardless of how upset or violated they may feel.
Set your exposue, lift your camera, focus n fire.
If the person seems reasonably calm n curious... Offer a print... But then keep moving if your subject is looking for an argument... If they get boligerant... Call a cop n not argue in the steert or then you can and will get yourself in hot water.