Voldemort. Professor Moriarty. Doctor Moreau. Mordred. The Morlocks. Aside from being nefarious figures, these characters have something in common: the syllable "mor." Is there something that makes that particular sound come off as evil to English-fluent ears?
Over at The Week, linguist James Harbeck examines the syllable "mor" and why it turns up so frequently in fictional things we associate with evil. He notes that "mor" doesn't necessarily imbue names (for example, Maureen, Morgan, Mork) with a sinister quality all by itself:
But we can say that it has some dark associations available if we want to use them. For starters, the Latin "mor" root (as in moribund and mortal and French words such as morte) refers to death; there is an old Germanic root mora for darkness, which shows up in words such as murky; our modern word murder comes from an Old English word morth for the same; and, of course, a morgue is a place where dead bodies are kept. That's enough to give a familiar ring. And every evil name that has "mor" in it adds to the weight of the association, especially when they're famous evil names.
In fact, "mor" may be what is sometimes called a phonestheme: a part of a word that tends to carry a certain connotation not because of etymology or formal definition but just by association. Words that start with "gl" often have to do with light (glow, gleam, glimmer, glitter, glisten, etc.) even though they are not all related historically; similarly, words that start with "sn" often relate to the nose (snoot, sniffle, snot, snore, sneeze, etc.). It doesn't mean that all words with those letters have the meaning in common, but there is a common thread among a notable set of them.
He continues to look into how phonesthemes occur, and how various fictional characters and locations got their particular "mor" sounds, adding weight to the idea that "mor" has become an evil-sounding syllable by virtue of becoming a phonestheme.