I think that the reason fang was prophesied to die was..
So in Maximum Ride there could only be one Max one person destined to save the world. So Max can only have only have one True lover. mga kerubin voice thought that Max was going to chose Dylan so the voice and Angel thought that Fang would have to die. When Max 2 left Maxs Life neither of them had to die so when Fang left Max at the end of FANG neither Fang nor Dylan had to die so if Dylan had left Max then Max being with Fang would not have been dangerous.
A bit complicated but makes sense.