From a utilitarian moral philosophy standpoint killing the empress was not evil if Daud thought it could save Dunwall or make Dunwall better/safer due to stronger leadership. However the Regent is on the extreme opposite side of the empress. It is a simple "The dosage makes the poison" comparison as a too kind/naive leader is not good for a country, but a selfish and cruelt tyrant sure as hell doesent work either. A good middleground is needed and imo neither the empress or the Regent was fit to rule. But this personal viewpoint is, I admit, a weak argument. Because you dont get to know them well enough to take an educated guess on a final outcome if they both got the chance unaffected by Corvo as a factor.
Edit: As an example, the empress could have succeeded. And the regent too even though his methods are inhumane and cruel. If you take maximum survivors as the "best" outcome then killing off and isolating the plague could have saved more than how the empress wouldve solved it (long term) but that doesent mean that the empress wouldnt have been able to deal with it aswell. But how I see it the empress was unable to make the tough decisions needed. The problem with the regent was ofcourse that he went too far and seemed to take personal pleasure in ridding the city of the poor as saving Dunwall was only secondary to him.
Evil and good isnt black and white, but complex. And the law does not by any means equal "Good". If you think following the law is good, and all who break it is evil - then you should see/read Les Miserables.
The statement in bold would be acceptable if it wasn't the lord regent that brought the plague to Dunwall in the first place to kill the poor. as for Daud i would place him in a Lawful Neutral alignment in D&D terms. To him, when the outsider gave him his powers, assassination was just his way to survive. It was all he knew how to do so he did it well, but the fact that he can look back and see what he has done and has remorse because of it, is the reason i spared him.
Another thing i found interesting about Daud is that he also regrets what has happened to Corvo. In his journal upstairs he talks about how he and Corvo are from the same island country, and he kind of identifies with Corvo. it also seems like he was forced into being an assassin by society and the Outsider.
At least that's what I gathered from his books and what he says.