As the title says, I think betrayal should be interpreted differently. You must be wondering why, right?
If we reread the dialogue between Cassie and Sunny, we'll see that Sunny asks how they would deal with the first seeker, and Cassie says that they have users with divine aspects and enhanced memories, And I think there were other arguments, but I don't remember them, but the important point is that Cassie HERSELF stated that they managed to deal with the third nightmare without him; he would have just made it easier.
From what she said, we can infer that Sunny's betrayal wasn't abandoning his friends, but rather the fact that he chose to live without them because he was pursuing what was rightfully his!
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