Ran through the quest on my Demon Slayer real quick, since the dialogue isn't in the data for some reason, too lazy to crop, including Mastema's dialogue as well, you can see it here:
http://imgur.com/a/L13oh
Basically, Demon says it was hard growing up as a demon among humans, he says he heard Masteria fell because they weren't united under a single, powerful ruler, and he joined the Black Mage so he wouldn't have to see that happen. Then he resolves not to follow another monster like the Black Mage again, and then decides to fight to protect what he has, rather than to advance a grand ideal that he may not be able to see in the end and addresses his assertion to his brother.
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