Remember in episode 4. A station with enough firepower to destroy a whole planet is unheard of.

+ there is that thing Mikkelsen said at the Rogue One panel...He said that his character had created something wonderful, that turned out not to be used in wonderful ways. Something like that. Classic scientist creating something exceptional that ends up being used as a weapon. Given the plot of the movie, him being involved with the Death Star's creation seems very very likely.