This question requires a little bit of background, so follow me on a little trip.
For some reason, the Empire needed Galen to complete the Death Star. He was a vital component to its completion due to the fact that he was one of the Galaxy's most brilliant minds in the field of crystals.
Galen had long since become disillusioned by the Empire and wanted nothing to do with them, choosing instead to live in the middle on nowhere with his wife and child.
Representatives of the Empire land on Galen's planet, try to convince him to help them finish the Death Star. By the end of this encounter, Galen's wife is dead and his child has been secreted away by a mad man, and Galen has agreed to help finish building the single most deadly weapon in the galaxy.
There are multiple problems with this. What leverage does the Empire have over Galen at this point? His wife is dead and no-one knows where his daughter is. There is nothing else Galen cares about, so why does he help the Empire?
Furthermore, it is clear that Galen is against the idea of building a giant genocide cannon, otherwise he would not have worked the weakness into the design...but why did he bother doing anything? One could argue that he feared for his own life, but that explanation does not fit with the fact that Galen was willing to risk being caught sabotaging the Death Star designs.
By not doing nothing, Galen contributed to the deaths of billions of people, and it could have all been avoided if he just said "no".
So, in short, What reason did Galen have to help the Empire finish designing and building the Death Star?