I read it as Snape being happy that Voldemort offered Lily an out: his deal with Voldemort must have been “please let Lily live and I’ll do anything”. The memory confirms the story that Voldemort gave Lily an option out (“stand aside”). So he considers Voldemort to have held up his side of the bargain.
This interpretation does not bode well for the Dumbledore-Snape alliance (which already seemed to be in bad shape in MoR)
It’s debatable whether Heisenberg did the former, causing the mistaken experiment results that led the Nazi atomic program to conclude that a bomb wasn’t viable. See http://en.wikipedia.org/wiki/Copenhagen_(play) for scientific entertainment (there’s a good BBC movie about this starring Daniel Craig as Werner Heisenberg)