>B) If we don't build it, someone else will. And we don't want this tech to fall into our enemy's hands first, do we? Best defense is a good offense.

I'm not actually clear on what's incorrect about this logic -- unless there's some kind of agreement across all superpowers not to research this subject, or the subject itself is not actually beneficial to research.. why would you assume that none of your potential adversaries are researching it?

And once you recognize that they might research it, and that if they did research it, and you didn't, they might switch from potential adversary to actual adversary (as they now believe themselves to have the upper hand), the only moral and ethical thing to do ... is to research it yourself!

Perhaps the more moral and ethical thing is to give away the technology once you have it, so that you yourself won't have the chance to be aggressor ... but otherwise, I'm not sure how you can justify not researching it (war, and the potential for it, exists regardless of whether you want it to)