Traditionally, mad scientists cackle and gibber and have wild, freaky hair. Dr. Murchison's hair is well-kempt and he keeps the gibbering to a minimum. Still, he's a nasty, evil mad scientist, and you shouldn't trust him with a test tube. Or a pistol, for that matter.
Murchison's the administrative head dude of Green Manors, a psychiatric institute. But like most psychiatrists in this movie, he's got some mental health issues himself (that's irony, folks). He had a mental breakdown from overwork, and the Board has decided that
MURCHISON: […] having crumbled once, I might crumble again.
In fact, Murchison is more than just overworked. He's kind of a psychopath. Angry at being ousted in favor of Dr. Edwardes, he goes out and kills Edwardes, which you have to admit is not the sort of thing a sane psychiatrist should do.
The character of the mad scientist, like good old Victor Frankenstein, is a way of expressing anxiety or worries about science. Everybody likes science; it makes the lights go on and gives us measles vaccines. But at the same time, science is kind of freaky. And scary. And maybe will create robots that kill us all.
Mad scientists: you can't trust 'em.
Spellbound seems to be all enthusiastic about psychiatry. The text at the beginning of the movie states that:
Once the complexes that have been disturbing the patient are uncovered and interpreted, the illness and confusion disappear…and the devils of unreason are driven from the human soul.
That all sounds great—but then you've got Dr. Murchison over there, and he's not banishing devils. He's shooting people, because his work as a psychiatrist unbalanced him.
Murchison doesn't have a lot of screen time; he's not much of a character. Instead, he's kind of an anxious shadow—a fear that maybe this psychiatry stuff doesn't banish unreason, but sneaks more unreason in.
Maybe, just maybe, science isn't clearing the cobwebs from the human mind. Maybe it's putting some creepy spiders in there, to skitter around between your ears.