Since this is not a Hitler situation (if you kill Hitler when he was a baby, someone else, maybe worse may take his place), why not go back to before he became immortal?
I thought that was what Rip did in the opening scene. After all, we saw that the palace/temple/whatever was still intact when Rip went after Hath-Set/Savage, and IIRC, the Flash/Arrow crossover showed that the building was destroyed by meteoroids in the event that gave Savage his immortality and the Hawks their reincarnation cycle.