During my thoughts, I will assume that Krennic is a relatively smart guy, since he somehow managed to move up the ladder so far, that he was mainly responsible for constructing the first Death Star.
Also, Galen Erso is nothing short of a genius and Krennic is well aware of this.
At some point between the events of RotS and the opening scene of Roque One, Erso finds that Krennic and the empire does not plan to use his reasearch for peaceful purposes but to construct weapons. Consequently he leaves the empire and hides from it. This is a very clear statement of Erso, that he is not going to construct weapons for the empire.
Then in the opening scene of Roque one, Krennic has Erso's wife killed and explicitly tells his troopers to do the same (or at least capture her) with his daugther Jyn:
Krennic: "They have a child. Find it!"
To get things straight: Galen decided to spend the rest of his life separated from civilazation only with his wife and daughter, so he really loved them. Then Krennic shows up, kills his wife, wants to capture/kill his daughter (which Galen predicted and thus bulit a hideout for her) and then forces him to work on something, which he explicitly refused to do at some earlier point.
Taking all this into consideration, preparing some inside job to spoil the Death Star was pretty much the obvious choice for Galen. Why on earth would Krennic trust him all this years between the opening scene of Roque One and the events shortly before Galen's death? He seems not to suspect anything bad from Galen until the point where Tarkin tells him that there are rumors about a traitor in Galen's team of scientists.
He could have hired someone to watch over Galen. This is a pretty common method in dictatorships, so we can safely assume that the empire also used to do that. But Krennic did not.
It is also explicitly stated, that the empire does not necessarily need Galen to finish the construction of the Death Star. So the argument that Krennic had no choice is somehow not working here.