If you'd read it carefully, you would have picked up on the fact that the dreamers in the house of beds is a metaphor for death. The fact that Lilith elected to join them at the end of the story implies precisely the opposite of your contention: i.e. that she chose death over empty physical immortality.
Lewis - and Christian orthodoxy - maintains that nothing is created ontologically evil. All that is created is good; but beings with free will - i.e. beings that can love - can choose not to love. I do it all the time. That's my dark, rebellious will in action. God didn't create that; I do it myself every time I choose a will other than His. Does that make me "evil"? Yes, it does - but I'm not ontologically evil. I'm created as an imagebearer of God, though damaged by sin.
What you're doing here, JtW, is called "
eisegesis". You've got your pet theory in place, and you interpret everything Lewis (or anyone else) wrote in light of that theory. You're bringing your interpretation to the work and viewing the work through that interpretation. Small surprise that no matter where you turn or what you read, you should see what you expect to see. It's like looking at everything through tinted glasses - of course everything looks pink if your glasses are rose. You're certainly free to do that, but don't expect the rest of us to see the same things you do, because we're not wearing the glasses.