It is said that the difference between a good man and a bad one is the choice of their cause, but that’s only partially true, as the perspective of the victor is the interpreter of all history.
I do not believe that Sir Croix is a bad man. He may truly believe that he is fulfilling his duty as a Knight of the Flame to protect the people of Alcaire by attacking the Redguards. Or perhaps like Sir Hughes, he may have fallen under the insidious influence of another.
The point is, it doesn’t matter his reasons or his cause; in the end the only difference between a good man and a bad one is whether or not he wins the day.