It's above my paygrade too, but what I remember from occasional discussions of that case is that:

- The obvious take is, the evil deed's already been done, the knowledge it produced can save lives and can't realistically be re-gathered any other way, so why not use it?

- The counter to that is, using it legitimizes and encourages similar acts in the future.

(Personally, I can see the encouragement angle; disagree with legitimization.)

- There's often a side thread going on about how the atrocities and those who committed them were not Up To Scientific Standards, therefore all their data is invalid, so there's no reason to use it anyway.

(Personally, I think this is a lame cop-out, used when one feels the ethical argument is too weak to stand on its own.)