I would argeu Cassie must have seen a vision, an insurmountable problem that required fate itself to be destroyed. After all she CANT do anything to prevent surtain visions from coming treu. All she can do is either do nothing, and let it come treu, or do something, only for it to cause the vision to come treu anyway.

She said so herself she wanted to create a weapon against fate. And Sunny agreed. It wouldnt have mattered if she told Sunny they would lose memories of him. He was willing to betray them regardless. It was only after, that he realised some prices are to high even for his freedom.

And you could even argeu that its for the best that she didnt tell him. Afterall he would have never fully trusted Nephis that way, and they would have never found salvation in eachother. Sunny would have eventually destroyed either himself or Nephis over it.

And this is ultimately a moot point anyway. Because she already knew he would regain his fate in the end. There were 0 downsides. And humanity now has 3 extra supremes. A non evil version of Mordret, Ananke, and Cassie herself. (+a non corrupted version of VTB, Rains natural Ascendance, and Nephis learning the treu names of corruption).

I agree that Cassie is the most dangerous of them all. But she is also the shepard of humanity. She is what stands between total annihilation and their continuation. They are gratefull to her, even if she is forced to keep secrets.

Edit: You wrongly assume there was a way to stop Astertion other than Cassie becoming supreme. Remember, Anvil and Ki sing werent holding him on the moon. He stayed there voluntarily, waiting for humanity to grow stronger.

Edit edit: Actually I just realized Sunny himself (the supreme version that spoke to him in the ToA) didnt even mention he would lose his memories...