If you want to get the good ending, you must not use Dark powers throughout the entire game. But if you have used them already and don't want to replay the whole game again you can:

in C:\Users\[computer name]\AppData\Local\XBlades find the newest save data file and modify it with a text editor. Change "corrupted:b=yes" to "corrupted:b=no". If you don't know which save file is the newest one, modify all saves the same way just to be sure.

Enjoy!