One does not simply use the letters 'mor' to show something is evil

Well, actually, you can. [Link]

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.

How does this happen? Whether it's through sound association or the force of a particular root word, it just seems to snowball. It may be partly through words with phonesthemes in them being preferred to words without (glitter chosen over coruscate because it sounds more, well, glittery), partly through words with phonesthemes in them shifting sense under the influence of the phonestheme (snub is getting more nose-focused), and partly through words changing form to come to have phonesthemes in them.

One possible case of a word changing form to have a phonestheme is the oldest of the "mor" names above, Mordred, the betrayer of King Arthur. His name actually was originally Medraut orModred, Celtic versions of the Latin Moderatus. How did it get the "mor"? Possibly with some influence of his mother, Morgause, or of Morgan le Fay. But possibly also through some sound associations, with murder (earlier murther) and with the French morte. After all, the best-known account of the Arthurian legend is Malory's Le Morte d'Arthur.

We know that some of the names drew directly and deliberately on "mor" words. J.R.R. Tolkien was a professor of Anglo-Saxon and knew very well what he was up to when he chose his words.Morgoth, Mordor, and Moria are all formed using the same mor root that shows up in his Elvish languages Quenya and Sindarin, a root referring to darkness and blackness. He borrowed it from the old Germanic word mora, which, as I mentioned, shows up in the modern word murky.

J.K. Rowling is well known as a dab hand at wordplay. Voldemort is right from French: it can mean "flight from death" or "theft of death." Rowling herself pronounces it with a silent t as in the French, though she's just about the only one who does.

Classical roots very likely played a role in the name Morbius, too. The first one, after whom the others (in Spider-Man and Dr. Who) are named, was in the 1956 movie The Forbidden Planet: Dr. Edward Morbius, his ship's master of languages and meaning, a man with an out-of-control unconscious. Morbius himself would have noticed the resemblance of his name to Möbius (of the famous loop) and Morpheus (shape-shifting god of dreams). He probably also would have known its similarity to Latin morbus, meaning "sickness" — source of English morbid. We can reckon safely that Irving Block and Allen Adler, who wrote the story an
d invented the name, had some idea of this too.