Not sure how you got that from the article. It says

In Tolkien’s Christian framework, [being open to morality, like Men] in turn meant they must have souls, so killing them would be wrong without very good reason.

It’s not them thinking and feeling that’s the problem, it’s that Tolkien’s believed that you shouldn’t kill things with souls.

This is a nice contrast to Frieren where demons don’t have souls and don’t seem to be open to morality, either; they understand it in order to manipulate humans but consider themselves outside its framework.