i need help with my German shepherd pup.?

okay well i live in Zambia and ive had my beloved female Bella for 7 months now . but ever since she was 4 months we would go out on the road and take a walk and everything would be fine . so there was this one time (when she was still 4 months) i went for a walk at night and she saw something in the distance of course before i did and she started barking and growling and i would try distract her but she would keep an eye on this thing until i figured out it was a person. to be specific he was a homeless man but he seems mentally disturbed . she kept on barking and growling while this man just looked at me and started whispering which was creepy. anyway every since that night she does the same thing to this homeless man when ever she sees him. and to be honest he hasn't done anything to her all he was doing was walking. she still does this all the time and she literally wont stop barking at him .....i always try and stop her but wont stop till i drag her away. she never does this with any other person even when walking at night. please don't leave nasty comments i just want to know whats wrong with her?