Well, speaking only for myself, it's clear that I'm not allowing myself to fully conceptualize the costs of a millenium of torture, even if I were able to, which I don't think I actually am.
But it's also clear that I'm not allowing myself, and am probably unable, to fully conceptualize the benefits of an immortal enjoyable life.
To put this more generally: my preference is to avoid cost/benefit tradeoffs where I can neither fully appreciate the costs nor the benefits. But, that said, I'm not sure that being able to appreciate the costs, but not the benefits, is an improvement.
Leaving all that aside, though... I suspect that, like you, I would choose to die (A) rather than choosing a long period of torture for someone else (B) if A and B were my choices. Of course, I don't know for sure, and I hope never to find out.
But I also suspect that if I found myself in the position of already having chosen B, or of benefiting from that choice made on my behalf, I would avoid awareness of having made/benefited the choice... that is, I suspect I am not one of those who actually walks away from Omelas.
I'm not proud of that, but there it is.
I have been trying to absorb the Lesswrong near-consensus on cryonics/quantum mechanics/uploading, and I confess to being unpersuaded by it. I'm not hostile to cryonics; just indifferent, and having a bit of trouble articulating why the insights on identity that I have been picking up from the quantum mechanics sequence aren't compelling to me. I offer the following thought experiment in hopes that others may be able to present the argument more effectively if they understand the objection here.
Suppose that Omega appears before you and says, “All life on Earth is going to be destroyed tomorrow by [insert cataclysmic event of your choice here]. I offer you the chance to push this button, which will upload your consciousness to a safe place out of reach of the cataclysmic event, preserving all of your memories, etc. up to the moment you pushed the button and optimizing you such that you will be effectively immortal. However, the uploading process is painful, and because it interferes with your normal perception of time, your original mind/body will subjectively experience the time after you pushed the button but before the process is complete as a thousand years of the most intense agony. Additionally, I can tell you that a sufficient number of other people will choose to push the button that your uploaded existence will not be lonely.”
Do you push the button?
My understanding of the Lesswrong consensus on this issue is that my uploaded consciousness is me, not just a copy of me. I'm hoping the above hypothetical illustrates why I'm having trouble accepting that.