Who overthrew the Shang dynasty, and how was this overthrow justified

To be exact, it was overthrew by its last emperor and the Emperor Wuwang, Jifa.
In ancient China, Chinese people believed in constellation and the heaven deeply and star moving shows that it emperors will occur in Zhou and Heaven will support him to do it. So based on it, the Zhou started their overthrown till its success.

I think you had a very good answer and It was the last emperor that over threw the Shang Dynasty.