Just don't read if your ego is too fragile, stop acting like cheating is not a thing in real life. Better go read some trash harem so that you could self-insert all you want and satisfy that fragile ego of yours.
I mean if it was his disiciples, they would certainly kill her. But if you understand his character and the progression, killing her sounds like the most pointless thing to do, has gotten pretty much everything he could wish for when the world got patched.
You clearly don't understand an introvert, pretty reasonable since nothing bad has happened to him. The threatening letter itself is pretty laughable as well from his POV, not sounding realistic and real juvenile-like, and making not much sense to him since he doesn't even have a GF or a girl he...
Haha, feels like a sequal to TEIS, with Cid finally clears the world and somehow returns to his old world, only to get sent back there again after finally growing out of his chuni phase.