|A Fairytale for the Demon Lord may suit as well, and it has romance in it, too.
Actually plot is like that;
Spoiler (mouse over to view)
A nameless knight saves a princess, but then king(?) took princess from him to marry her, but she was the knight's everything. So, the knight tries to get her back but gets corrupted on the way to the princess
It is a very sad story, too. And a webtoon in colors^^